GIFT   OF 


ELEMENTS 
OF  AVIATION  ENGINES 


ELEMENTS  OF 

AVIATION 
ENGINES 

By  JOHN  B.  F.  BACON,  PH.  B. 

Instructor,  Engines  Department 

U.  S.  School 

of  Military  Aeronautics 
Berkeley,  California 


PAUL  ELDER  AND  COMPANY 

SAN  FRANCISCO  M  CM  XVIII 


COPYRIGHT,  1918,  BY 

fi-  W          JOHN  B.  F.  BACON 
BERKELEY,  CAL. 


CONTENTS 

PAGE 

INTRODUCTION       /    .      .      • VII 

CHAPTER  I 

THE  AVIATION  ENGINE      .      . 3 

CHAPTER  II 

APPLICATION  OF  THE  BASIC  PRINCIPLE  .       .'      .       .       7 

CHAPTER  III 
ENGINE  SPECIFICATIONS .16 

CHAPTER  IV 

ENGINE  PARTS      ....      .      ,      .      .V    .     22 

CHAPTER  V 

CARBURETION      ;JY      ....      ...      .       .     47 

CHAPTER  VI 

IGNITION.      >      ,;     ," ; '    .    •..      ^      .     ,„      .      .     57 

CHAPTER  VII 

LUBRICATION  .      .      .     '^      .      .      ,.      .      .      ,     71 

CHAPTER  VIII 

COOLING  .      .      .      .      .      ,      .      -   ^-      -      ••    80 

CHAPTER  IX 

ROTARY  ENGINES  .       .      .      .      .      .      .      .      .     84 

CHAPTER  X 

THE  LIBERTY  MOTOR        .      .'. 96 

INDEX  .   105 


III] 


388382 


ILLUSTRATIONS 

FACING  PAGE 
THRUST  BEARINGS 36 

DIAGRAM  TO  ILLUSTRATE  THE  CURTISS  Ox  VALVE 
ACTION .     42 

THE  MILLER  AVIATION  CARBURETOR    ....     50 
A  HALF  SECTION  VIEW  OF  A  ZENITH  CARBURETOR     .     52 

DIAGRAMS  TO  ILLUSTRATE  THE  LOCATION  OF  THE  CORE 
IN  A  SHUTTLE  TYPE  MAGNETO 58 

WIRING  DIAGRAM  OF  A  MAGNETO  SYSTEM  .       .       .62 

DIAGRAM  TO  ILLUSTRATE  THE  PRINCIPLE  OF  REVOLV- 
ING POLES  ON  THE  DIXIE  MAGNETO  ....  64 

DIAGRAM  TO  ILLUSTRATE  POSITION  OF  ROTOR  IN  THE 
DIXIE  MAGNETO  WHEN  THE  CORE  is  MAGNETIZED  66 

DIAGRAM  TO  ILLUSTRATE  POSITION  OF  ROTOR  IN  THE 
DIXIE  MAGNETO  WHEN  THE  CORE  is  DEMAGNE- 
TIZED .  . .  66 

DIAGRAM  OF  A  BATTERY  SYSTEM  OF  IGNITION  WITH  A 
NON  VIBRATING  COIL .  68 

GEAR  PUMP 76 

DIAGRAM  TO  ILLUSTRATE  THE  OPERATION  OF  A  VANE 
PUMP .76 

CENTRIFUGAL  PUMP 82 

DIAGRAM  TO  ILLUSTRATE  THE  PRINCIPLE  OF  A 
ROTARY  ENGINE  84 


IVJ 


INTRODUCTION 

HAVING  been  forcibly  impressed  with  the 
fact  that  many  of  those  who  take  up  the 
study  of  aviation  are  not  familiar  with  gasoline 
engines  and  have  little  mechanical  inclination, 
it  has  been  the  endeavor  of  the  writer  to  explain 
in  a  simple  way  some  of  the  points  that  appear 
to  cause  beginners  the  greatest  amount  of  trouble. 
While  it  may  aid  those  who  are  conscientiously 
reviewing  the  subject,  it  is  far  from  the  purpose 
of  this  book  to  provide  a  short  cut  to  passing 
marks  on  examination  papers. 

All  of  the  information  herein  contained  has 
been  before  the  engineering  public  at  one  time  or 
another.  Realizing  that  certain  new  develop- 
ments must  not  appear  in  print  during  this 
critical  period  every  precaution  has  been  taken 
to  observe  strict  avoidance  of  revealing  confiden- 
tial information. 

The  writer  wishes  to  express  his  gratitude  to 
the  members  of  the  Engines  Department  in  the 
S.  M.  A.  of  Berkeley  for  their  assistance.  Special 
thanks  is  due  Mr.  James  Irvine  for  his  sugges- 
tions which  have  resulted  in  many  improvements. 

JOHN  B.F.BACON, 
818th  Aero  Depot  Squadron,  U.  S.  A. 


Berkeley,  Cal.,  August,  1918. 

[VII 


ELEMENTS 

OF 

AVIATION  ENGINES 


ELEMENTS 
OF  AVIATION  ENGINES 

CHAPTER  I 
THE  AVIATION  ENGINE 

IN  TAKING  up  a  new  subject  it  is  often  best 
to  fix  clearly  in  mind  just  what  is  meant  by 
the  name  of  the  subject,  so  in  beginning  a  dis- 
cussion upon  aviation  engines  it  seems  well  to 
start  with  a  rough  definition  of  the  term  avia- 
tion engine.  A  simple  statement  that  an  in- 
ternal combustion  engine  so  designed  that  it  is 
capable  of  lifting  from  the  ground  and  sustain- 
ing in  flight  a  heavier  than  air  flying  machine 
will  suffice  as  a  definition  for  our  subject.  By 
the  term  internal  combustion,  engine  is  com- 
monly meant  simply  a  gasoline  engine,  because 
in  such  an  engine  the  power  is  derived  from  the 
force  of  an  explosion  within  a  cylinder.  This 
will  make  clear  what  we  mean  by  our  subject. 
The  question  at  once  arises :  Why  must  avia- 
tion engines  be  internal  combustion  engines  in- 
stead of  steam  engines,  and  why  not  propel 
aeroplanes  by  aid  of  electricity?  The  answer  is 
simply  that  maximum  power  and  minimum 

[3] 


;  ;;  CLEMENTS  OF  AVIATION  ENGINES 

weight  can  be  best  obtained  with  the  internal 
combustion  engine.  In  the  study  of  aeronau- 
tics weight  is  a  tremendous  factor,  and  it  is  in- 
teresting to  note  that  not  until  the  gasoline 
engine  had  reached  its  modern  development 
was  human  flight  practical.  On  account  of  the 
unlimited  use  of  gasoline  as  a  motive  power 
and  the  increasing  interest  of  technical  men  in 
the  problems  of  aviation,  the  gasoline  engine 
has  been  developed  to  such  a  point  that  it  may 
deliver  1  H.P.  for  every  1.8  pounds  of  its 
weight.  To  a  mechanical  mind  this  seems  one 
of  the  greatest  achievements  of  the  twentieth 
century. 

Since  gasoline  engines  have  been  used  so  ex- 
tensively and  with  such  marked  success  in 
automobiles,  the  aviation  student  will  at  once 
involuntarily  compare  the  aviation  engine  with 
that  in  an  automobile,  and  oftentimes  he  com- 
pares them  wrongly  by  stating  that  the  avia- 
tion engine  develops  a  vastly  greater  speed 
than  the  engine  of  an  automobile  is  capable  of 
attaining.  This  is  incorrect  and  is  a  poor  way 
of  comparing  the  two.  The  main  difference  is 
that  of  lightness.  Aviation  engines  are  of  the 
lightest  possible  construction  and  are  designed 
to  run  continuously  at  their  highest  speed. 

[41 


THE  AVIATION  ENGINE 

Seldom  are  the  frail  supporting  members  for 
the  engines  in  a  horizontal  plane,  and  often  the 
engine  is  called  upon  to  do  its  work  while  com- 
pletely inverted.  These  are  conditions  that  the 
automobile  engine  does  not  have  to  meet.  In 
order  to  attain  a  construction  that  will  fulfill 
the  requirements  imposed  upon  aviation  en- 
gines, it  is  natural  to  expect  that  some  sacri- 
fice must  be  made.  This  accounts  for  their  low 
degree  of  durability.  When  we  examine  the 
heavy  construction  of  a  400  H.P.  marine  gaso- 
line engine  and  then  regard  the  frail  parts  of  a 
400  H.P.  aviation  engine  there  is  not  the 
slightest  doubt  which  engine  will  continue 
longer  in  its  operations.  However,  since  light 
construction  is  an  absolute  necessity,  it  is  use- 
less to  expect  much  in  the  way  of  durability, 
and  as  a  means  of  knowing  what  an  aviation 
engine  will  stand  it  is  interesting  to  note  that 
after  every  seventy-five  hours  of  operation  the 
engine  should  be  rebuilt. 

As  a  compact  and  light  power  plant  the  avia- 
tion engine  is  the  highest  attainment  of  me- 
chanical genius.  It  has  been  developed  from 
the  type  that  propels  the  automobiles,  and 
just  as  the  old  types  of  automobile  engines  do 
not  resemble  in  appearance  the  types  used  to- 

[5] 


ELEMENTS  OF  AVIATION  ENGINES 

day,  so  the  first  aviation  engines  have  little 
resemblance  to  those  of  the  present  time.  The 
development  has  been  rapid,  and  it  is  difficult 
to  predict  what  will  be  the  effect  upon  aviation 
if  the  rapid  strides  taken  during  the  past  ten 
years  continue  to  add  to  the  efficiency  and  re- 
liability of  the  aviation  engine  during  the  next 
ten  years  to  come. 


[6] 


CHAPTER  II 

APPLICATION  OF  THE  BASIC 
PRINCIPLE 

THE  WORKING  principle  of  an  aviation  en- 
gine is  identically  the  same  as  that  of  the 
ordinary  gasoline  engine.  In  the  middle  of  the 
nineteenth  century  it  was  satisfactorily  proven 
that  the  explosive  force  of  gasoline  could  be 
used  to  actuate  a  piston,  and  this  has  given 
rise  to  the  adoption  of  a  new  form  of  motive 
power.  Since  that  time  gasoline  engines  have 
been  developed  along  two  lines,  one  being 
called  the  two-stroke  cycle  engine,  and  the 
other  the  four-stroke  cycle  engine,  but  since  the 
former  has  not  been  used  extensively  in  aviation 
work  little  attention  will  be  given  to  it  here. 
A  two-stroke  cycle  engine  is  one  in  which  an 
explosion  takes  place  in  the  cylinder  every 
time  the  crank  shaft  makes  one  revolution.  A 
charge  of  combustible  gas  is  slightly  com- 
pressed within  the  crank  case  by  the  piston 
traveling  downward.  Near  the  bottom  of  this 
downward  stroke  the  piston  uncovers  a  port  in 
the  cylinder  wall  allowing  some  of  the  com- 
pressed gas  to  enter  the  cylinder.  Then  the 

[7] 


ELEMENTS  OF  AVIATION  ENGINES 

piston  moves  upward,  closing  the  port  and 
compressing  the  gas.  The  charge  is  ignited 
when  the  piston  is  near  the  end  of  its  upward 
stroke,  and  the  result  is  that  the  force  of  the 
explosion  violently  drives  the  piston  down- 
ward. An  exhaust  port  on  the  opposite  side  of 
the  cylinder  from  the  intake  port  is  uncovered 
as  the  piston  sweeps  downward,  and  the  force 
of  the  explosion  starts  the  burnt  gas  rushing 
out  of  the  cylinder.  The  intake  port  having 
also  been  uncovered  by  this  time  will  allow  a 
fresh  charge  to  enter.  By  using  a  deflector  on 
the  piston  head  the  fresh  charge  is  hindered 
from  rushing  straight  to  the  exhaust  port  and 
is  diverted  upward,  serving  admirably  to  expel 
the  remaining  burnt  gases.  Now  the  piston  is 
ready  to  go  upward  again,  and  the  same  opera- 
tions are  repeated.  In  this  way  the  piston 
makes  two  strokes  to  complete  a  cycle,  hence 
it  is  spoken  of  as  the  two-stroke  cycle  engine. 

Some  confusion  may  be  caused  by  not 
knowing  the  exact  meaning  of  the  word  cycle, 
so  it  may  be  well  to  insert  here  a  definition.  A 
complete  series  of  events  occurring  in  regular 
sequence  and  ending  so  that  the  same  opera- 
tion can  be  repeated  in  the  same  order  is  called 
a  cycle. 

[8] 


APPLICATION  OF  THE  BASIC  PRINCIPLE 

The  four-stroke  cycle  engine  has  proven  the 
more  satisfactory  of  the  two  types,  and  since 
it  is  the  one  used  in  connection  with  aviation, 
it  is  very  desirable  to  fully  understand  it.  This 
type  differs  from  the  two-stroke  cycle  in  that 
it  has  two  distinct  mechanically-operated 
valves  in  the  cylinder  which,  of  course,  necessi- 
tate a  few  more  working  parts.  Instead  of  the 
gas  being  stored  and  compressed  within  the 
crank  case,  this  engine  draws  its  explosive 
charge  directly  from  the  carburetor  by  opening 
the  inlet  valve  as  the  piston  goes  downward 
and  making  use  of  the  suction  thus  exerted. 
The  charge  is  compressed  by  the  reversal  of  the 
piston's  motion  and  the  closing  of  the  inlet 
valve.  Near  the  end  of  this  compression  stroke 
the  charge  is  ignited,  resulting  in  an  explosive 
force  being  exerted  on  the  piston  when  it  is 
ready  to  go  downward  again.  Near  the  end  of 
this  succeeding  downward  stroke  the  exhaust 
valve  is  opened  permitting  the  force  of  the  ex- 
plosion to  give  the  burnt  gases  their  initial 
outward  impulse.  The  valve  remains  open 
during  the  entire  upward  stroke  of  the  piston 
to  insure  all  of  the  burnt  gases  being  expelled. 
The  clearing  out  of  the  cylinder  is  often  re- 
ferred to  as  scavaging  the  cylinder.  Generally 

[9] 


ELEMENTS  OF  AVIATION  ENGINES 

the  exhaust  valve  closes  after  the  piston  has 
reached  its  uppermost  position.  This  brings  us 
to  the  opening  of  the  inlet  valve  and  with  that 
the  sequence  of  events  is  repeated. 

By  the  stroke  of  the  piston  is  meant  the 
movement  of  the  piston  in  one  direction.  It 
follows  from  this  that  the  length  of  the  stroke 
is  the  linear  distance  the  piston  travels  from 
its  uppermost  position  to  its  lowest  position  or 
vice  versa.  The  term  stroke  has  come  to  mean 
simply  the  number  of  inches  between  top  cen- 
ter and  bottom  center,  thus  designating  the 
two  extreme  positions  of  the  piston.  To  make 
clear  the  four  strokes  of  the  piston  in  a  four- 
stroke  cycle  engine,  the  first  one  in  which  the 
piston  goes  down  and  draws  in  a  charge  is 
called  the  intake  stroke.  The  next  upward 
motion  is  the  compression  stroke.  Then  comes 
the  explosion  which  drives  the  piston  down- 
ward. This  is  the  power  stroke.  Finally  the 
expulsion  of  the  burnt  gases  is  the  exhaust 
stroke,  and  this  completes  the  cycle. 

In  aviation  engines  it  is  customary  to  ignite 
the  charge  near  the  end  of  the  compression 
stroke  instead  of  at  the  beginning  of  the  power 
stroke.  The  speed  of  the  engine  justifies  this. 
If  ignition  were  to  take  place  when  the  piston 

[10] 


APPLICATION  OF  THE  BASIC  PRINCIPLE 

was  at  top  center  or  a  little  afterward,  the 
force  of  the  explosion  would  be  exerted  upon 
the  piston  head  at  such  a  late  time  that  the 
piston  could  not  deliver  its  maximum  impulse 
to  the  crank  shaft.  When  the  piston  is  nearing 
bottom  center  its  effectiveness  for  transmitting 
force  is  negligible.  Consequently  by  opening 
the  exhaust  valve  at  the  end  of  the  power 
stroke  instead  of  at  the  beginning  of  the  ex- 
haust stroke,  the  force  of  the  explosion  serves  to 
start  the  burnt  gases  rushing  outward  without 
losing  power.  The  exhaust  valve  is  generally 
held  open  until  the  beginning  of  the  intake 
stroke.  This  aids  in  scavaging  the  cylinder  as 
it  permits  more  time  for  the  operation,  and  the 
danger  of  retaining  some  of  the  burnt  gases 
is  avoided  since  the  out-going  exhaust  will 
possess  a  certain  amount  of  inertia.  Different 
makes  of  engines  have  different  times  for  open- 
ing the  intake  valve.  On  some  there  is  a 
small  interval  between  the  closing  of  the 
exhaust  valve  and  the  opening  of  the  inlet 
valve,  as  is  the  case  with  the  Curtiss  OX  and 
the  Hall-Scott.  This  permits  the  downward 
motion  of  the  piston  to  establish  somewhat  of 
a  rarefication  within  the  cylinder,  so  that  when 
the  inlet  valve  is  opened  there  will  be  a  ten- 

[111 


ELEMENTS  OF  AVIATION  ENGINES 

dency  for  the  gas  to  enter  more  promptly. 
The  closing  of  the  inlet  valve  occurs  at  the  be- 
ginning of  the  compression  stroke.  The  gas 
passing  through  the  manifold  will  have  some 
inertia  which  will  maintain  a  flow  into  the 
cylinder  during  the  first  part  of  ensuing  up- 
ward stroke.  By  thus  keeping  the  valve  open 
past  bottom  center  a  larger  amount  of  gas  is 
placed  in  the  cylinder. 

The  question  often  arises:  Why  are  not  two- 
stroke  cycle  engines  used  for  aviation  work,  on 
account  of  the  decrease  in  weight  due  to  the 
less  number  of  working  parts,  the  more  fre- 
quent power  impulses,  and  the  need  of  an 
engine  that  will  do  its  best  work  when  running 
at  top  speed?  The  two-stroke  cycle  engine  ful- 
fills all  the  requirements  demanded  of  an 
aviation  engine  except  for  the  fact  it  will  not 
ordinarily  run  satisfactorily  at  low  enough 
speeds  to  allow  the  propeller  to  idle.  Since  a 
successful  aviation  engine  must  be  able  to  run 
slow  enough  without  stopping  to  allow  the 
plane  to  glide,  it  can  be  easily  seen  that  the 
present  form  of  two-stroke  cycle  engine  is 
poorly  suited  for  aviation  work. 

So  far  in  explaining  the  different  operations 
involved  in  a  cycle,  only  one  cylinder  has  been 

[12] 


APPLICATION  OF  THE  BASIC  PRINCIPLE 

considered.  It  is  advisable  to  have  frequent 
power  impulses  and  to  avoid  vibration  as 
much  as  possible.  This  is  accomplished  by 
using  a  number  of  cylinders  which  decreases 
the  weight  of  the  reciprocating  parts. 

Vibration  is  due  to  the  shifting  of  the  cen- 
ters of  gravity  of  pistons  and  connecting  rods. 
In  a  single  cylinder  engine  of  required  power 
turning  at  a  speed  suitable  to  drive  a  propeller, 
the  amount  of  vibration  would  be  prohibitive. 
The  greatest  bearing  pressure  in  an  engine  at 
high  speeds  comes  not  so  much  from  the  ex- 
plosion, but  from  the  effort  of  starting  and 
stopping  the  weight  of  the  piston  and  connect- 
ing rod.  To  decrease  this  reciprocating  weight 
it  is  necessary  to  resort  to  the  basic  law  of 
volumes  and  areas.  If  we  make  a  body  half  the 
dimensions  of  another,  it  will  have  but  one 
quarter  of  the  area  and  only  one-eighth  of  the 
weight.  This  can  be  applied  to  pistons.  Thus  a 
piston  can  be  replaced  by  four  smaller  ones  half 
as  large,  and  the  area  of  the  four  will  equal  that 
of  the  larger  one.  However,  these  four  pistons 
will  weigh  practically  one-half  as  much  as  the 
original  single  piston.  This  illustrates  the  way 
reciprocating  weight  is  lessened  and  shows  plain- 
ly the  demand  for  a  larger  number  of  cylinders. 

[13] 


ELEMENTS  OF  AVIATION  ENGINES 

The  way  the  cylinders  are  arranged  serves  as 
a  means  of  classifying  aviation  engines.  If  the 
cylinders  stay  in  a  fixed  position  in  respect  to 
the  crank  shaft,  it  is  spoken  of  as  a  fixed  cylin- 
der engine,  but  if  the  cylinders  revolve  about 
the  crank  shaft  it  is  called  a  rotary  engine. 
Various  difficulties  in  construction  are  encoun- 
tered when  the  number  of  cylinders  is  increased, 
so  fixed-cylinder  engines  are  not  confined  to 
the  vertical  style  but  are  often  built  in  a  V 
form  to  permit  a  shorter  crank  shaft.  A  pecu- 
liar style  of  fixed-cylinder  engine  is  that  with 
an  additional  row  of  cylinders  between  the 
two  rows  that  go  to  make  up  the  V.  This  is  the 
design  of  the  Sunbeam  Engine.  Another  style 
of  fixed-cylinder  engine  is  one  in  which  the 
cylinders  radiate  from  the  crank  case  allowing 
the  force  of  all  explosions  to  be  exerted  upon 
the  same  crank  pin.  The  Anzani  engine  is  of 
this  design.  The  rotary  engines  have  not  so 
many  variations.  As  a  means  of  increasing  the 
number  of  cylinders  a  second  bank  of  cylinders 
is  often  added,  which  of  course  necessitates 
two  throws  on  the  crank  shaft.  Rotary  engines 
are  limited  to  those  having  one  and  two  banks. 
In  both  the  fixed-cylinder  and  the  rotary  types 
the  growing  demand  for  an  increased  number 

[14] 


APPLICATION  OF  THE  BASIC  PRINCIPLE 

of  cylinders  has  resulted  in  the  adoption  of  en- 
gines of  the  designs  just  referred  to  for  aviation 
work. 


[15] 


CHAPTER  III 
ENGINE  SPECIFICATIONS 

A  A  BASIS  of  comparing  aviation  engines 
certain  specifications  are  used.  It  must  be 
remembered  that  all  engines  are  not  called  up- 
on to  do  the  same  work,  and  furthermore  that 
they  are  not  all  designed  by  one  man  or  even 
by  a  group  of  men  holding  the  same  views  on 
various  mechanical  problems.  This  will  ac- 
count for  the  wide  range  in  specifications.  In 
order  to  become  familiar  with  the  points  where 
engines  differ,  a  few  items  will  be  taken  up  here. 

The  first  point  to  consider  is  whether  the 
engine  has  fixed  cylinders  or  is  a  rotary.  If  it 
is  a  fixed-cylinder  engine,  the  arrangement  of 
the  cylinders  should  be  noted.  Generally 
speaking,  rotary  engines  are  used  for  very  fast 
but  brief  flights,  while  fixed-cylinder  engines 
serve  better  for  long  flights  where  speed  is  not 
so  important. 

The  horse-power  of  an  engine  is  probably  the 
matter  of  greatest  interest.  All  planes  are  not 
of  the  same  size  and  weight,  so  there  is  need 
for  engines  of  different  power.  One  horse- 
power is  the  power  required  to  lift  33,000 

[16] 


ENGINE  SPECIFICATION 

pounds  a  distance  of  one  foot  in  one  minute. 
The  horse-power  necessary  to  operate  a  plane 
is  calculated  by  multiplying  the  total  air  re- 
sistance of  the  plane,  expressed  in  pounds,  by 
the  speed  in  feet  per  second,  then  by  60  sec- 
onds in  a  minute,  and  dividing  the  product  by 
33,000.  The  actual  horse-power  that  an  engine 
develops  is  spoken  of  as  brake  horse-power. 
It  may  be  found  by  measuring  the  torque 
exerted  by  the  engine  running  with  a  propeller 
attached.  By  torque  is  meant  the  moment  of 
tangential  effort,  or  to  put  it  more  roughly,  a 
force  tending  to  produce  rotation.  The  torque 
is  allowed  to  be  exerted  upon  an  arm  which 
delivers  the  force  to  a  platform  balance.  By 
multiplying  the  force  in  pounds  by  the  dis- 
tance in  feet  through  which  it  acts  in  one  revo- 
lution by  the  R.P.M.  and  dividing  the  product 
by  33,000,  the  actual  horse-power  is  obtained. 
The  distance  through  which  the  force  acts  is 
the  circumference  of  a  circle  having  the  power 
arm  as  a  radius.  This  distance  will  be  6.2832 
times  the  arm's  length,  so  if  we  make  the  arm 
exactly  5J4  feet  long,  the  distance  through 
which  the  force  acts  will  be  33  feet.  This  per- 
mits us  to  reduce  our  fraction  to  the  lowest 
terms,  making  the  denominator  1,000  instead 

[17] 


ELEMENTS  OF  AVIATION  ENGINES 

of  33,000.  The  horse-power  can  then  be  ob- 
tained by  multiplying  the  torque  expressed  in 
pounds  by  the  R.P.M.  and  then  dividing  by 
1,000,  which  simply  amounts  to  moving  the 
decimal  point  three  places  to  the  left. 

The  weight  of  an  engine  is  of  great  import- 
ance, for  it  determines  the  engine's  fitness.  As 
has  been  said  before,  aviation  work  requires 
maximum  power  for  minimum  weight.  Light- 
ness is  the  keynote  of  the  whole  engine,  so  the 
aviation  engine  is  devoid  of  all  unnecessary 
equipment.  Self-starters  are  seldom  used  on 
account  of  their  weight  and  mufflers  never,  on 
account  of  their  weight  and  resistance  also. 
Aviation  engines  avoid  the  use  of  a  fly-wheel, 
on  account  of  the  large  number  of  cylinders 
and  also  on  account  of  the  steadying  effect  of 
the  propeller.  In  speaking  of  the  weight  of  an 
engine,  the  weights  of  tanks  and  radiators  are 
not  included,  nor  does  oil  or  water  enter  into 
the  engine's  weight.  By  dividing  the  weight 
by  the  horse-power  the  weight  per  horse-power 
is  obtained.  This  is  a  very  significant  figure 
and  is  widely  used  in  comparing  engines.  The 
most  modern  types  of  aviation  engines  range 
from  two  to  three  pounds  in  weight  for  every 
horse-power  developed. 

[18] 


ENGINE  SPECIFICATION 

The  speed  of  most  aviation  engines  is  gener- 
ally about  1,400  R.P.M.  being  a  compromise 
between  the  most  efficient  propeller  speed  and 
the  most  efficient  engine  speed.  An  ordinary 
propeller  will  do  its  best  work  when  turning 
from  900  to  1,000  R.P.M.  If  it  is  driven  con- 
siderably faster  than  that,  it  will  cause  what 
is  known  as  cavitation,  which  means  that  the 
blades  are  working  in  an  unfavorable  medium 
so  far  as  their  usefulness  is  concerned.  This 
will  show  the  undesirability  of  having  pro- 
pellers turn  at  speeds  which  a  high-grade  auto- 
mobile motor  can  easily  attain.  Consequently 
since  the  speed  of  an  engine  is  normally 
greater  than  900  or  1,000  R.P.M.  it  is  advis- 
able to  compromise  by  driving  the  propeller  a 
little  faster  than  it  ought  to  turn  and  running 
the  engine  at  a  reduced  speed.  The  efficiency 
of  an  engine,  which  roughly  speaking  is  the 
proportion  between  the  energy  received  as 
work  and  the  energy  supplied  as  fuel,  can  be 
increased  if  the  engine  is  permitted  to  run 
faster  than  1,400  R.P.M.  Since  the  propeller 
speed  has  limitations,  engines  running  at 
higher  speeds  must  have  a  gear  reduction  re- 
garding the  propeller.  This  is  ordinarily  ac- 
complished by  driving  a  jack  shaft  carrying 

[191 


ELEMENTS  OF  AVIATION  ENGINES 

the  propeller  by  spur  gears  one  above  the 
other.  Sometimes  internal  gears  are  used  and 
then  the  propeller  will  turn  in  the  same  direc- 
tion that  the  engine  turns. 

The  disadvantages  of  a  geared  propeller  are 
that  more  weight  is  added  and  a  slight  amount 
of  power  is  consumed  by  the  gears. 

The  direction  of  rotation  of  an  engine  should 
be  considered.  When  standing  directly  in  front 
of  the  propeller  and  noting  that  it  turns  coun- 
ter-clockwise, the  engine  is  spoken  of  as  having 
a  normal  rotation.  Should  the  propeller  turn 
clockwise  the  engine  has  an  anti-normal  rota- 
tion. One  reason  for  building  both  normal  and 
anti-normal  engines  is  that  in  case  a  plane  has 
two  engines  as  is  sometimes  the  case  with 
bombing  planes,  then  normal  and  anti-normal 
engines  are  used  to  equalize  the  torque  effect. 

The  number  of  cylinders  and  their  bore, 
meaning  the  internal  diameter,  is  an  important 
item.  The  stroke  of  the  piston  which  has  been 
mentioned  before  is  often  spoken  of  in  connec- 
tion with  the  bore.  Various  engines  use  differ- 
ent strokes  with  different  bores,  but  for  the 
sake  of  illustration,  the  stroke  averages  about 
one  and  one-quarter  times  the  bore.  If  both 
the  bore  and  the  stroke  are  large,  there  will  be 

[20] 


ENGINE  SPECIFICATION 

a  tendency  to  develop  heat  on  the  compression 
stroke  providing  the  compression  chamber  is 
small.  The  total  piston  displacement  is  calcu- 
lated by  squaring  half  the  bore,  multiplying  by 
3.1416,  then  multiplying  by  the  stroke,  and 
finally  by  the  number  of  cylinders.  The  result 
will  be  in  cubic  inches.  The  horse-power  per 
cubic  inch  of  piston  displacement,  which  is 
obtained  by  dividing  the  horse-power  by  the 
displacement,  is  a  figure  of  much  interest. 
Efficient  motors  will  give  from  .17  to  .27  H.P. 
for  each  cubic  inch  of  displacement. 

Ignition,  carburetion,  and  cooling  enter  into 
the  specifications  of  an  engine,  but  since  separ- 
ate chapters  are  devoted  to  them  later,  they 
need  not  be  dealt  with  here. 


21 


CHAPTER  IV 
ENGINE  PARTS 

r  I  lo  TAKE  up  all  the  parts  of  an  engine  and 
JL  describe  them  fully  would  be  a  big  under- 
taking, and  might  not  prove  interesting  to 
those  beginning  this  subject.  Consequently 
only  the  principal  parts  will  be  included  and 
dealt  with  in  a  very  brief  manner. 

The  cylinders  of  a  gasoline  engine  are  vari- 
ously constructed.  They  may  be  made  as  in- 
dividual units,  or  several  may  be  cast  in  block. 
The  advantage  of  the  former  method  of  con- 
struction is  that  more  complete  jacketing  can 
be  accomplished,  while  rigidity  is  the  advan- 
tage of  the  latter  type.  In  case  an  engine  had 
four  cylinders  cast  in  block  and  one  became 
damaged,  then  the  three  good  ones  would  have 
to  be  discarded  in  order  to  replace  the  one 
cylinder  that  caused  the  trouble.  This  waste 
is  not  encountered  when  each  cylinder  is  a 
separate  replaceable  unit.  However,  from  the 
standpoint  of  compactness  the  block  construc- 
tion is  by  far  the  more  preferable.  Individual 
cylinders  are  made  of  cast  iron,  semi-steel,  and 
steel.  When  cast  in  block  their  material  is 

[22] 


ENGINE  PARTS 

usually  aluminum  alloy.  A  peculiar  form  of 
construction  is  that  used  in  the  Curtiss  cylin- 
ders, where  each  cylinder  is  of  cast  iron  with  a 
band  of  some  non-corrosive  metal  such  as 
monel  metal  to  act  as  a  water  jacket.  The 
cylinders  of  the  Hispano-Suiza  are  unusual  in 
design,  being  steel  thimbles  that  screw  into  an 
aluminum  alloy  water  jacket  designed  to  hold 
four  cylinders.  The  Sturtevant  cylinders  are 
interesting  in  that  they  are  of  aluminum  alloy 
cast  in  pairs  with  a  steel  liner  shrunk  in  to  act 
as  a  cylinder  wall. 

The  location  of  the  valves  determines  the 
shape  of  the  cylinder  head.  If  the  valves  oper- 
ate in  extensions  on  opposite  sides  of  the  com- 
bustion chamber  the  cylinder  is  said  to  have  a 
T  head,  since  its  shape  is  that  of  a  T.  This  con- 
struction necessitates  two  independent  cam 
shafts  besides  being  rather  bulky,  so  is  of  little 
importance  from  the  standpoint  of  aviation 
work.  If  a  cylinder  has  only  one  extension  in 
which  a  valve  or  valves  work,  its  shape  will 
resemble  that  of  a  Greek  letter  gamma  or  sim- 
ply an  inverted  L.  It  is  therefore  called  an  L 
head.  When  a  cylinder  has  no  extensions  on 
either  side  but  has  two  valves  located  in  its 
head,  it  is  called  an  I  head  cylinder.  This  type 

[23] 


ELEMENTS  OF  AVIATION  ENGINES 

of  cylinder  is  the  most  popular  for  aviation 
engines,  because  it  does  away  with  an  irregu- 
larly-shaped combustion  chamber.  In  the  case 
of  a  T  or  L  head  cylinder  the  space  above  the 
valves  may  be  regarded  as  a  pocket,  and  very 
often  it  is  difficult  to  scavage  these  pockets. 
The  placing  of  both  valves  in  the  head  permits 
the  combustion  chamber  to  be  made  slightly 
spherical  in  order  to  reduce  the  surface  area 
and  lessen  the  amount  of  heat  carried  away  at 
the  time  when  an  explosion  takes  place. 

Some  cylinders  are  made  so  that  the  head 
may  be  removed  without  disturbing  its  base. 
This  is  known  as  a  detachable  head  and  has 
the  advantage  of  providing  an  easy  means  of 
removing  carbon  and  working  upon  the  valves. 
However,  a  little  more  material  is  required  in 
this  construction,  and  it  brings  into  account 
compression  leaks  and  also  water  leaks  since 
the  cylinder  heads  must  be  jacketed. 

The  crank  case  is  generally  divided  into  two 
parts;  the  top  section  serving  as  a  base  for  the 
cylinders  and  the  bottom  section  carrying  a 
supply  of  oil.  The  sump  is  that  part  which 
holds  the  oil.  As  a  rule  crank  cases  are  alumi- 
num castings,  and  in  case  the  motor  is  a  V  type 
great  care  is  taken  to  strengthen  the  upper  sec- 

[24] 


ENGINE  PARTS 

tion  by  means  of  partitions  or  webs  to  prevent 
the  strain  exerted  by  explosions  on  opposite 
banks  from  cracking  the  upper  section.  The 
crank  shaft  bearings  are  generally  held  in  the 
upper  section.  Sometimes  the  lower  halves  of 
the  bearings  are  held  in  partitions  in  the  lower 
section  of  the  crank  case,  as  in  the  Hispano- 
Suiza.  The  difficulty  in  this  construction  is 
that  the  lower  section  can  not  be  removed 
without  disturbing  the  crank  shaft.  As  a 
means  of  retaining  the  oil  in  the  sump  when 
the  engine  is  momentarily  inverted,  splash 
pans  are  placed  in  the  lower  section.  They  do 
not  retain  all  of  the  oil,  but  aid  in  reducing  the 
amount  that  would  otherwise  rush  into  the 
cavities  of  the  pistons.  The  vents  on  crank 
cases  are  called  breathers.  These  maintain 
atmospheric  pressure  in  the  crank  case  even 
though  compression  leaks  are  present. 

That  part  of  the  engine  which  is  driven 
downward  within  the  cylinder  by  the  force  of 
an  explosion  is  the  piston.  Pistons  have  re- 
ceived as  much  if  not  more  attention  by  de- 
signers than  any  other  part  of  the  engine,  and 
the  result  has  been  to  secure  satisfactory  oper- 
ation at  high  speeds  and  at  high  temperatures. 
The  material  used  in  piston  construction  is 

[25] 


ELEMENTS  OF  AVIATION  ENGINES 

generally  aluminum  alloy,  although  cast  iron 
is  sometimes  used.  The  use  of  aluminum  as 
piston  material  serves  to  lessen  vibration  and 
increase  the  speed,  lessening  the  weight  of  re- 
ciprocating parts.  Another  reason  for  its  use 
is  the  rapidity  with  which  it  conducts  heat. 
The  piston  head  may  be  either  convex,  flat,  or 
concave,  and  all  of  these  shapes  are  in  use  at 
present.  The  convex  or  domehead  brings  into 
account  the  ability  of  an  arch  to  withstand 
strain.  Greater  strength  for  a  given  amount  of 
material  is  obtained  by  using  a  convex  head. 
The  flat  head  is  the  common  type.  By  having  a 
flat  surface  less  area  of  the  piston  is  exposed  to 
absorb  heat.  This  results  in  a  slightly  cooler  pis- 
ton, which  is  a  big  advantage,  as  it  is  impossible 
to  cool  the  piston  in  the  same  way  that  the  cyl- 
inder is  cooled.  The  concave  head  has  been  ex- 
tensively used  on  rotary  engines  because  it 
permits  a  shorter  cylinder  and  thus  lessens  the 
centrifugal  force.  This  shape  of  piston  head  al- 
lows the  combustion  chamber  to  assume  a  spher- 
ical form.  By  the  bosses  are  meant  the  two 
projections  within  the  piston  that  hold  the  wrist 
pin,  and  it  follows  that  the  upper  end  of  the  con- 
necting rod  must  fit  between  the  two  bosses. 
The  lower  portion  of  a  piston  is  termed  the  skirt. 
[26] 


ENGINE  PARTS 

Due  to  more  material  at  the  head  and  also 
on  account  of  the  top  surface  coming  in  direct 
contact  with  the  heat  of  each  explosion,  it  will 
be  seen  that  the  upper  part  of  the  piston  will 
expand  more  than  the  skirt.  This  necessitates 
allowing  more  clearance  between  the  cylinder 
wall  and  the  piston  at  its  head  than  at  its 
skirt.  Some  idea  of  this  difference  can  be  had 
by  pointing  out  that  a  five-inch  piston  may  be 
cleared  .020  inch  at  the  skirt  and  as  much  as 
.027  at  the  head. 

To  prevent  compression  and  the  force  of  an 
explosion  from  passing  down  between  the  pis- 
ton and  the  cylinder  wall,  piston  rings  are 
used.  These  fit  in  grooves  in  the  piston  and 
bear  upon  the  cylinder  wall.  Besides  prevent- 
ing leaks  these  rings  prevent  much  oil  from 
getting  upon  the  piston  head  where  it  would 
result  in  the  formation  of  carbon.  The  rings 
are  made  of  cast  iron,  and  each  piston  gener- 
ally requires  two  or  three  of  them.  When  the 
two  ends  of  a  ring  come  together  squarely 
the  ring  is  said  to  have  a  butt  joint.  When  the 
ends  meet  each  other  diagonally  it  is  called  a 
diagonal  joint.  Likewise  if  the  ends  are  made 
so  that  they  meet  each  other  in  the  form  of  a 
step,  it  is  called  a  step  or  lap  joint.  Obviously 

[271 


ELEMENTS  OF  AVIATION  ENGINES 

a  ring  having  a  step  joint  will  offer  more  resis- 
tance to  the  passage  of  gas  than  those  having 
butt  or  diagonal  joints.  A  precaution  to  take 
in  placing  a  piston  in  a  cylinder  is  to  make  sure 
the  joints  in  the  rings  are  at  equal  intervals 
around  the  circumference  of  the  piston. 

The  wrist  pin  is  made  of  steel,  usually  hol- 
low and  case  hardened,  and  is  used  to  form  a 
movable  joint  between  the  piston  and  the  con- 
necting rod.  Its  length  depends  upon  the 
diameter  of  the  piston.  There  are  three  gen- 
eral ways  of  retaining  the  pin  in  its  right  posi- 
tion. It  may  be  held  rigidly  in  the  connecting 
rod  by  means  of  a  clamp  or  a  set  screw  which 
results  in  the  pin  turning  in  the  piston  bosses 
as  the  connecting  rod  moves  back  and  forth. 
This  method  is  used  in  the  Curtiss  OX.  An- 
other way  is  to  pin  the  wrist  pin  in  the  bosses 
so  that  it  is  securely  held  in  a  fixed  position. 
The  connecting  rod  will  then  turn  on  the  wrist 
pin  which  means  the  bearing  will  be  in  the 
connecting  rod.  Such  a  construction  necessi- 
tates a  bearing  at  both  ends  of  the  connecting 
rod.  The  Hall-Scott  ASA  has  its  wrist  pins 
held  rigidly  in  the  piston  bosses.  The  floating 
method  such  as  used  in  the  Sturtevant  allows 
the  pin  to  move  either  in  the  bosses  or  in  the 

[28] 


ENGINE  PARTS 

connecting  rod.  Brass  ends  on  the  pin  or  caps 
over  the  ends  of  the  bosses  protect  the  cylinder 
walls. 

The  connecting  rod  is  a  steel  arm  used  to 
convert  the  reciprocating  motion  of  the  piston 
into  the  revolving  motion  of  the  crank  shaft. 
The  majority  of  connecting  rods  have  a  cross 
section  resembling  an  I,  although  H  and  tubu- 
lar rods  are  not  uncommon.  In  cases  where 
the  wrist  pin  is  held  in  the  bosses  the  upper  end 
of  the  connecting  rod  is  supplied  with  a  bronze 
bushing  that  acts  as  a  bearing  surface.  The 
lower  bearing,  in  which  works  the  crank  pin, 
is  given  more  attention.  Babbitt  is  employed 
as  a  bearing  metal  and  is  generally  backed  by 
bronze  to  take  its  place  should  enough  heat  be 
developed  to  fuse  the  babbitt.  Lower  connect- 
ing rod  bearings  are  made  in  two  pieces  to 
permit  the  crank  pin  being  put  in  position. 
Between  the  two  halves  of  the  bearing  are  placed 
strips  of  metal  called  shims.  These  are  .001, 
.002  and  .005  inch  thick  and  as  the  bearing 
wears  away  these  can  be  removed  insuring  a 
better  fit.  In  a  V  motor,  when  the  vertical  axis 
of  opposite  cylinders  are  in  the  same  vertical 
plane,  the  connecting  rods  of  opposite  cylin- 
ders will  meet  the  crank  pin  at  the  same  point. 

[29] 


ELEMENTS  OF  AVIATION  ENGINES 

This  will  necessitate  the  forked  or  straddled 
construction  in  which  one  rod  works  between 
the  fork  of  another.  It  makes  rather  a  com- 
plicated and  costly  bearing,  but  it  is  a  favorite 
design  and  is  being  used  extensively.  The 
Hispano-Suiza  has  this  type  of  lower  connect- 
ing rod  bearings.  Another  and  simpler  way  is 
to  have  the  cylinders  "staggered"  by  placing 
the  cylinders  on  one  bank  a  little  ahead  or  be- 
hind those  on  the  opposite  bank,  thereby 
allowing  two  lower  connecting  rod  bearings  to 
work  side  by  side  on  one  crank  pin.  A  wise 
precaution  to  take  in  assembling  a  motor  is  to 
make  sure  the  lower  connecting  rod  bearing  is 
such  that  it  allows  the  wrist  pin  to  be  abso- 
lutely parallel  with  the  crank  pin.  If  it  is 
otherwise  the  piston  will  not  work  freely  within 
the  cylinder. 

The  crank  shaft  is  the  driving  shaft  of  the 
engine  to  which  the  power  impulses  are  trans- 
mitted by  the  pistons  and  connecting  rods. 
It  is  needless  to  say  that  this  is  the  most  im- 
portant moving  part  of  an  engine,  and  for  this 
reason  it  is  made  with  great  precision  from 
selected  pieces  of  high-grade  steel  by  drop 
forging  and  subsequent  turning.  The  principal 
parts  of  a  crank  shaft  are  the  main  bearings, 

[30] 


ENGINE  PARTS 

the  cheeks,  and  the  pins  on  which  the  connect- 
ing rods  work.  Two  cheeks  and  a  pin  are 
spoken  of  as  a  throw.  Thus  the  number  of 
cylinders  govern  the  number  of  throws,  and 
also  upon  the  number  of  cylinders  depends  the 
number  of  degrees  between  the  throws.  In  a 
vertical  motor,  if  the  cylinders  are  cast  separ- 
ately, there  is  generally  a  main  bearing  be- 
tween every  two  throws.  Where  the  cylinders 
are  cast  in  block  there  is  not  so  much  space 
between  the  pistons  which  often  means  a  de- 
crease in  the  number  of  main  bearings.  The 
crank  shaft  used  in  a  V  motor  is  identically  the 
same  as  one  used  in  a  vertical  motor  having 
half  the  number  of  cylinders.  Two  connecting 
rods  are  fitted  to  each  throw,  and  if  the  cylin- 
ders are  cast  separately  a  main  bearing  is 
placed  between  every  two  throws. 

For  the  main  bearings  of  a  crank  shaft  the 
lining  is  babbitt  usually  backed  by  bronze  very 
similar  to  the  lower  connecting  rod  bearings. 
Babbitt,  which  essentially  consists  of  lead  and 
antimony,  is  used  as  bearing  metal  because  of  its 
anti-friction  properties,  its  sufficient  hardness, 
and  the  ease  with  which  it  can  be  replaced. 
Lead  alone  possesses  considerable  anti-friction 
properties,  but  is  impracticable  on  account 

[31] 


ELEMENTS  OF  AVIATION  ENGINES 

of  its  softness.  The  addition  of  some  anti- 
mony will  materially  harden  the  lead  with- 
out lessening  its  anti-friction  properties.  The 
use  of  babbitt  also  permits  the  liner  to  be 
scraped  to  secure  an  exact  bearing  surface. 
By  coating  the  journal  with  Prussian  blue,  the 
high  spots  can  be  detected  on  the  liner,  and 
these  can  be  successively  removed  by  scraping. 

To  have  evenly  placed  power  impulses  the 
throws  on  a  crank  shaft  must  be  placed  at  cer- 
tain angles  with  one  another.  In  any  four- 
stroke  cycle  motor  all  cylinders  will  fire  once 
in  two  revolutions  of  the  crank  shaft  or  once 
in  720  degrees.  In  a  four-cylinder  motor  there 
would  be  four  explosions  in  720  degrees,  and  to 
get  equal  spacing  the  power  impulses  would 
have  to  come  one-fourth  of  720  or  180  degrees 
apart.  This  will  explain  why  the  angle  be- 
tween two  throws  that  receive  impulses,  one 
directly  after  the  other,  is  180  degrees  for  a 
four-cylinder  crank  shaft.  The  throws  in  a  six- 
cylinder  crank  shaft  are  120  degrees  apart, 
since  there  will  be  six  power  impulses  in  720 
degrees. 

In  determining  the  order  in  which  the  cylin- 
ders will  deliver  their  power  impulses  to  the 
crank  shaft,  it  is  the  custom  to  fire  them  so 

[32] 


ENGINE  PARTS 

that  the  vibrations  set  up  by  one  explosion  will 
serve  to  counteract  the  vibrations  caused  by  a 
previous  explosion.  To  accomplish  this  an  ex- 
plosion at  one  end  of  the  shaft  is  followed  by 
an  explosion  near  the  other  end. 

Here  we  come  to  what  is  known  as  the  firing 
order,  which  simply  means  the  order  in  which 
the  cylinders  do  their  work.  In  order  to  discuss 
the  firing  order  it  is  first  necessary  to  explain 
how  the  cylinders  are  numbered.  In  American 
practice  cylinder  No.  1  is  always  that  one  at 
the  pilot's  end  of  the  engine,  and  the  number- 
ing is  in  regular  order  toward  the  propeller. 
In  V  engines  No.  1  is  the  first  cylinder  on  the 
left  bank  viewed  from  the  pilot's  cock  pit. 
Some  engines  have  the  left  bank  numbered 
LI,  L2,  L3,  L4,  and  the  right  bank  Rl,  R2, 
R3,  R4.  Others  number  the  left  bank  1,  2,  3,  4 
in  the  regular  way  and  then  start  with  the 
cylinder  nearest  the  propeller  on  the  right 
bank  calling  it  1'  followed  by  2',  3'  and  4' 
going  toward  the  pilot's  end.  The  Curtiss  OX 
has  the  peculiar  way  of  starting  with  No.  1  on 
the  left  bank  nearest  the  cock  pit  and  desig- 
nating as  No.  2  the  opposite  cylinder  on  the 
right  bank.  No.  3  is  the  next  one  on  the  left 
bank,  and  in  this  way  the  odd  numbers  are  on 

[33] 


ELEMENTS  OF  AVIATION  ENGINES 

the  left  bank  and  the  even  numbers  on  the 
right  bank. 

To  return  to  firing  orders,  it  is  best  to  start 
with  a  four-cylinder  engine.  The  cylinders  in 
such  an  engine  can  be  fired  in  a  1,  2,  4,  3  order 
or  in  a  1,  3,  4,  2  order.  From  this  it  can  be  seen 
that  throws  1  and  2  are  180  degrees  apart  and 
3  and  4  are  also  that  distance  apart.  Likewise 
it  is  evident  that  with  a  four-cylinder  crank 
shaft,  pistons  1  and  4  travel  together  and  also 
2  and  3  are  coming  up  or  going  down  together. 
The  two  usual  ways  for  a  six-cylinder  engine  to 
fire  are  1,  5,  3,  6,  2,  4,  and  1,  4,  2,  6,  3,  5.  Here 
the  throws  are  120  degrees  apart,  and  the  pis- 
tons that  travel  together  are  1  and  6,  2  and  5, 
and  3  and  4.  V  engines  use  the  basic  four- 
cylinder  and  six-cylinder  firing  orders  to  fire 
the  two  banks.  The  explosions  will  alternate 
between  the  two  banks  starting  with  the  cylin- 
der at  the  pilot's  end  on  the  left  block  and  fol- 
lowed by  the  forward  cylinder  on  the  right 
block.  Explosions  will  occur  on  the  left  bank 
according  to  either  one  of  the  two  firing  orders, 
and  those  on  the  right  bank  in  like  manner 
except  that  on  the  right  bank  we  will  be  work- 
ing from  the  propeller  end  toward  the  pilot's 
end.  Where  an  engine  is  numbered  LI,  L2, 

[34] 


ENGINE  PARTS 

etc.,  and  Rl,  R2,  etc.,  its  firing  order  may  be: 
LI,  R6,  L5,  R2,  L3,  R4,  L6,  Rl,  L2,  R5, 
L4,  R3. 

Where  the  left  bank  is  numbered  1,  2,  3, 
etc.,  and  the  right  bank  1',  2',  etc.,  in  the  oppo- 
site direction,  the  firing  order  may  be: 
11'    55'    3  V   66'   22'  44' 

1,1    ,    3,0   ,    O,O   ,    U,U   ,    Z,,Z,   ,   't,'*  . 

The  Curtiss  OX  with  its  peculiar  cylinder 
numbering  already  referred  to  has  the  follow- 
ing distinctive  firing  order  for  normal  rota- 
tion: 

1,  2,  3,  4,  7,  8,  5,  6. 

For  an  anti-normal  engine  it  would  be: 

2,1,4,3,8,7,6,5. 

or  to  start  the  cycle  with  an  explosion  in  cylin- 
der No.  1  it  would  be: 

1,4,3,8,7,6,5,2. 

In  order  that  the  thrust  exerted  by  the 
propeller  upon  the  crank  shaft  may  be  trans- 
mitted to  the  crank  case  and  then  to  the 
fuselage,  a  thrust  bearing  is  placed  upon  the 
crank  shaft  very  near  the  propeller  hub. 
Thrust  bearings  are  generally  ball  bearings  hav- 
ing either  one  or  two  rows  of  balls  and  very  often 
they  are  designed  to  take  a  load  directed  at 
right  angles  towards  the  center  of  the  shaft  as 
well  as  taking  care  of  the  thrust.  In  an  engine 

(351 


ELEMENTS  OF  AVIATION  ENGINES 

like  the  Curtiss  OX,  where  the  crank  shaft  ex- 
tends several  inches  between  the  last  main 
crank-shaft  bearing  and  the  propeller  hub,  the 
thrust  bearing  will  be  the  last  point  where  the 
shaft  may  be  supported.  Now  if  a  shaft  is 
allowed  to  revolve  without  a  radial  bearing  at 
its  end  vibration  will  result  and  this  must  be 
avoided.  Consequently  on  the  Curtiss  OX  and 
all  other  engines  having  a  nose,  the  thrust 
bearing  must  be  capable  of  taking  both  radial 
and  thrust  loads.  Some  thrust  bearings  having 
a  single  row  of  balls  will  only  take  thrust  in  one 
direction.  This  makes  it  necessary  to  reverse 
the  bearings  if  an  engine  is  transferred  from  a 
tractor  plane  to  a  pusher  plane  or  vice  versa. 

The  cam  shaft  is  that  part  of  the  engine  hav- 
ing irregularities  upon  its  surface  that  open 
and  close  the  valves  at  the  proper  time.  The 
irregularities  are  called  cams  and  are  usually 
accurately  shaped  projections  upon  the  shaft 
for  imparting  the  necessary  motion  to  a  valve. 
Cam  shafts  are  always  made  of  high-grade 
steel  and  the  cams  are  forged  integral  with  the 
shaft.  When  gasoline  engines  were  first  being 
developed  it  was  the  practice  to  have  as  a  cam 
shaft  a  plain  piece  of  shafting  with  the  cam 
keyed  or  pinned  to  it  in  the  right  position. 

[36] 


\ 


CURTI&S  OX 


•r 


THRUST  BEARINGS 


ENGINE  PARTS 

This  resulted  in  an  endless  amount  of  cam- 
shaft trouble  as  the  cam  would  often  come 
loose  causing  a  valve  to  operate  at  the  wrong 
time  or  possibly  not  operate  at  all.  Now  that 
the  cams  and  shaft  are  made  in  one  piece,  this 
difficulty  is  no  longer  encountered. 

The  location  of  the  cam  shaft  has  been  a 
matter  of  much  discussion.  The  old  practice 
was  to  have  it  located  at  the  base  of  the  cylin- 
ders as  this  was  the  most  convenient  position 
where  T  and  L  head  cylinders  were  used. 
Since  I  head  cylinders  are  more  favorably 
looked  upon,  the  overhead  position  of  the  cam 
shaft  is  being  used  more  and  more,  as  it  does 
away  with  the  numerous  push  rods  used  to 
operate  the  overhead  valves.  However,  a  cam 
shaft  so  placed  necessitates  a  pillar  shaft  and 
bevel  gears  to  drive  it.  V  engines  that  use  the 
base  position  of  the  cam  shaft  usually  have  the 
cylinders  placed  in  a  staggered  position.  This 
makes  it  much  easier  for  one  cam  shaft  located 
at  the  bottom  of  the  V  to  operate  the  valves 
on  both  banks  of  cylinders.  When  a  V  engine 
uses  the  overhead  position  two  cam  shafts  are 
necessary. 

In  all  four-stroke  cycle  engines  the  cam  shaft 
always  travels  at  half  the  crank-shaft  speed. 

[37] 


ELEMENTS  OF  AVIATION  ENGINES 

The  reason  is  that  it  takes  two  revolutions  of 
the  crank  shaft  to  complete  a  cycle  and  that 
the  individual  valves  must  open  but  once  dur- 
ing a  cycle.  For  instance,  one  cylinder  will  fire 
once  during  two  revolutions  of  the  crank  shaft. 
In  order  that  it  may  function,  an  inlet  valve 
must  open  once  to  let  a  new  charge  in.  Then 
the  intake  valve  will  open  once  during  two 
revolutions  of  the  crank  shaft  which  means 
that  the  cam  operating  that  valve  must  re- 
volve once  to  two  revolutions  of  the  crank 
shaft. 

Upon  the  valves  depend  to  a  great  extent 
the  success  of  the  engine,  for  aviation  engines 
seem  particularly  susceptible  to  valve  trouble. 
The  two  general  types  of  valves  for  gasoline 
motors  are  the  poppet  or  mushroom  type  and 
the  sliding  sleeve  type.  The  former  is  univer- 
sally used  for  aviation  work  largely  because  the 
latter  type  brings  into  account  a  little  more 
weight.  A  poppet  valve  consists  primarily  of 
a  disk  with  a  bevelled  edge  and  a  stem  joining 
the  disk  at  its  center.  The  bevelled  edge  is 
usually  at  an  angle  of  45  degrees  with  the  plane 
of  the  disk,  although  other  angles  are  not  un- 
common. By  having  the  valves  open  inwardly 
the  force  of  an  explosion  or  the  force  of  a  com- 

[38] 


ENGINE  PARTS 

pression  stroke  will  tend  to  push  the  valve 
firmly  against  the  bevelled  portion  of  the  cy- 
linder referred  to  as  the  seat,  and  in  this  way 
the  greater  the  force  within  the  cylinder  the 
more  tightly  will  the  valve  be  held  in  its  closed 
position.  It  is  safe  to  say  that  valves  in  avia- 
tion motors  should  be  as  large  as  possible.  The 
use  of  I  head  cylinders  restricts  the  size  of  the 
valves,  so  it  is  often  impossible  to  put  in  a 
valve  of  a  satisfactory  diameter.  T  and  L 
head  cylinders  permit  the  use  of  larger  valves 
on  account  of  the  extension  to  the  combustion 
chamber.  The  object  of  using  large  valves  is 
simply  to  charge  and  scavage  a  cylinder  more 
rapidly. 

When  we  consider  under  what  conditions 
the  valves  must  do  their  work,  it  will  be  seen 
why  a  great  deal  of  attention  has  been  paid  to 
the  materials  of  which  they  are  made.  The 
exhaust  valve  opens  on  the  power  stroke 
allowing  the  highly-heated  gases  to  escape 
around  it.  Particles  of  carbon  will  invariably 
be  carried  outward  and  some  will  at  times  be 
caught  between  the  valve  and  its  seat  at  the 
instant  it  closes.  The  valve  having  been  highly 
heated  on  account  of  its  direct  contact  with 
the  explosion,  will  be  somewhat  soft,  and  when 

[39] 


ELEMENTS  OF  AVIATION  ENGINES 

it  snaps  against  the  particle  of  carbon  a  small 
indentation  will  be  caused.  This  is  called  pit- 
ting, and  to  lessen  it  to  a  great  extent  it  has 
now  become  the  custom  to  make  the  exhaust 
valve  of  tungsten  steel.  Of  the  two  valves  the 
inlet  is  less  subject  to  pitting,  since  the  incom- 
ing gas  tends  to  cool  it,  and  furthermore  less 
carbon  collects  on  its  seat.  Nickel  steel  is  the 
material  sometimes  used  for  inlet  valves. 

In  order  to  make  a  valve  seat  more  firm  after 
an  engine  has  run  considerably,  and  to  prevent 
leaking,  it  is  necessary  to  grind  a  new  surface 
both  on  the  valve  and  its  seat  in  the  cylinder. 
The  abrasive  is  called  grinding  compound.  It 
is  applied  as  a  very  thin  paste  to  either  the 
valve  or  its  seat,  whereupon  the  valve  is  in- 
serted in  its  usual  position  and  vigorously 
turned  back  and  forth.  If  care  is  taken  to  fre- 
quently unseat  the  valve  the  compound  will 
be  kept  evenly  distributed  over  the  grinding 
surface,  and  there  will  be  little  danger  of  cut- 
ting rings  in  either  the  valve  or  its  seat.  Prus- 
sian blue  can  be  used  to  determine  the  fit. 
Frequently  a  valve  may  become  warped  or  a 
shoulder  may  develop  on  the  seat.  A  reamer 
can  then  be  used  to  good  advantage,  but  it 
must  be  followed  by  grinding.  Sometimes  the 

[40] 


ENGINE  PARTS 

guide  in  which  the  valve  stem  works  will  be- 
come worn,  making  it  useless  to  grind  a  valve 
until  a  bushing  or  new  guide  has  been  supplied. 

The  springs  used  to  close  the  valves  deserve 
attention.  On  some  engines  double-coil  springs 
are  used,  and  then  in  case  a  spring  breaks  there 
will  still  be  one  to  close  the  valve.  Occasion- 
ally the  exhaust  valve  springs  will  be  a  little 
heavier  than  those  on  the  inlet  valves.  This  is 
to  allow  for  any  decrease  in  strength  caused  by 
the  heat  from  the  exhaust  valve  and  also  to 
prevent  any  possibility  of  the  exhaust  valve 
being  pulled  down  on  the  intake  stroke. 

The  ways  in  which  the  force  of  a  revolving 
cam  is  brought  to  bear  upon  a  valve  stem  are 
numerous  and  interesting.  With  an  L  head 
cylinder  where  the  cam  shaft  runs  directly 
under  the  valve,  it  is  a  simple  matter  to  have  a 
follower  riding  the  cam  and  a  tappet  rod  be- 
tween the  follower  and  the  valve  stem.  When 
the  cam  comes  up  the  valve  will  be  pushed  up. 
As  the  cam  goes  on  the  spring  will  bring  the 
valve  back  to  its  seat.  This  is  simplicity  itself, 
but  the  use  of  I  head  cylinders  makes  neces- 
sary other  means  of  transmitting  the  cam 
thrust. 

The  usual  way  of  operating  valves  in  the 

[41] 


ELEMENTS  OF  AVIATION  ENGINES 

cylinder  head  by  a  cam  shaft  located  at  the 
base  of  the  cylinders  is  to  use  push  rods  con- 
nected to  rocker  arms  working  on  fulcrums 
attached  to  the  tops  of  the  cylinders.  As  the 
push  rod  is  forced  up,  one  end  of  the  rocker 
arm  goes  up  and  the  other  end  goes  down, 
pushing  inwardly  on  the  end  of  the  valve  stem. 
This  is  the  way  the  valves  are  operated  on  the 
Curtiss  VX  and  the  Sturtevant.  The  peculiar- 
ity in  the  Sturtevant  is  that  the  side  thrust 
imparted  to  tappet  is  avoided  by  having  a 
pivoted  arm  ride  the  cam  and  on  this  arm  rests 
the  tappet.  Worn  guides  are  reduced  to  a 
minimum  in  this  way. 

The  inlet  valve  operation  on  the  Curtiss  OX 
is  interesting  inasmuch  as  it  brings  into  ac- 
count a  new  form  of  cam,  and  also  because  the 
valve  is  pulled  open  instead  of  receiving  a 
direct  thrust.  Upon  the  cam  shaft  for  each 
inlet  valve  are  two  cams  that  would  be  com- 
pletely circular  but  for  a  flat  space  on  each. 
The  space  between  these  two  cams  is  taken 
up  by  the  exhaust  valve  cam,  which  is  the 
ordinary  type  of  cam.  Upon  the  two  round 
cams  rests  a  tappet  to  which  is  attached  a  rod, 
or  strictly  speaking  a  tube,  having  a  coil  spring 
held  about  it  at  the  top  by  a  strap  and  at  the 

[42] 


DIAGRAM  TO  ILLUSTRATE  THE  CURTISS  OX  VALVE  ACTION 


ENGINE  PARTS 

bottom  by  a  collar  upon  the  tube.  The  upper 
end  of  the  tube  is  hinged  to  a  lever  arm  that 
extends  almost  horizontally  from  a  fulcrum 
upon  the  head  of  the  cylinder,  and  directly 
under  this  lever  arm  is  the  end  of  the  inlet 
valve  stem.  As  the  cam  shaft  revolves  the  flat 
spaces  on  the  two  cams  will  allow  the  spring  to 
force  the  tappet  and  tube  toward  the  center 
of  the  cam  shaft,  which  results  in  a  downward 
motion  of  one  end  of  the  lever  arm.  Since  the 
inlet  valve  is  located  beneath  it  and  since  the 
spring  on  the  tube  is  several  times  stronger 
than  the  spring  upon  the  valve  stem,  the  inlet 
valve  is  thus  pulled  open  and  remains  open  as 
long  as  the  flat  spaces  will  permit  the  spring  to 
keep  the  tube  in  its  downward  position.  It 
will  be  noticed  that  in  this  manner  of  opening 
the  inlet  valve  everything  depends  upon  the 
spring  on  the  pull  tube. 

Making  the  valves  open  and  close  at  the 
right  time  on  an  engine  or  timing  the  valves,  as 
it  is  generally  called,  is  a  simple  matter  pro- 
viding it  is  done  systematically.  The  first 
thing  to  do  is  to  select  a  cylinder,  preferably 
No.  1  and,  making  sure  a  cam  is  not  in  a  posi- 
tion to  deliver  a  thrust,  adjust  the  clearance 
of  a  valve  on  that  cylinder  by  means  of  a  feeler 

[43] 


ELEMENTS  OF  AVIATION  ENGINES 

gage  so  that  the  clearance  is  that  given  by  the 
manufacturer.  Valve  clearance  varies  between 
.010  and  .030  inch,  and  its  purpose  is  to  allow 
for  expansion  of  the  stem  and  also  to  obtain  a 
very  accurate  adjustment  of  a  particular  valve. 
It  is  the  custom  to  time  on  the  opening  of  an 
intake  valve  or  on  the  closing  of  an  exhaust 
valve.  After  the  clearance  has  been  adjusted 
for  one  valve  and  after  the  cam  shaft  gears 
have  been  unmeshed,  the  engine  is  turned  in 
the  direction  it  is  intended  to  rotate  until  the 
piston  in  the  selected  cylinder  is  exactly  in  the 
right  position  for  the  inlet  valve  to  open  or  for 
the  exhaust  valve  to  close.  Then  the  cam  shaft 
is  revolved  by  hand  in  the  direction  it  is  in- 
tended to  turn  until  the  inlet  valve  is  just 
starting  to  open  or  the  exhaust  valve  has  just 
closed  as  the  case  may  be.  The  next  step  is  to 
mesh  the  cam  shaft  gears.  Sometimes  the 
teeth  come  directly  together  and  when  that  is 
the  case  it  is  necessary  to  "split  a  tooth." 
Different  engines  have  ways  of  doing  this,  but 
it  generally  amounts  to  providing  some  means 
of  revolving  the  gear  wheel  upon  the  cam 
shaft  the  distance  of  half  a  tooth,  which  is 
enough  to  allow  the  teeth  to  be  meshed  with- 
out disturbing  the  cam  shaft. 
[44] 


ENGINE  PARTS 

All  of  the  other  valves  on  the  engine  are 
timed  by  adjusting  the  clearance  for  each  one. 
A  piston  is  placed  in  the  right  position  for  a 
valve  to  open  or  close  and  if  it  does  not  func- 
tion correctly  the  clearance  is  changed  until  it 
opens  or  closes  on  time.  From  this  it  can  be 
seen  that  if  the  cam  shaft  is  out  of  time  all 
valves  will  be  affected,  while  if  the  clearance  is 
set  wrong  it  will  affect  only  the  valve  having 
the  wrong  clearance.  In  other  words,  the  cam 
shaft  affects  every  valve,  while  clearance 
affects  the  individual  valve.  In  cases  where 
too  much  clearance  is  given  above  a  valve  stem 
the  valve  will  open  late  and  close  early.  When 
there  is  too  little  clearance,  the  valve  will  open 
early  and  close  late. 

When  spark  plugs  are  placed  in  the  cylinder 
head  it  is  possible  to  determine  the  position  of 
a  piston  at  any  time  by  removing  a  plug  and 
inserting  a  steel  scale.  If  valves  are  timed  by 
determining  a  piston's  position  in  this  manner, 
it  is  spoken  of  as  the  linear  method  of  timing. 
Inaccuracies  may  result  from  the  use  of  this 
method  where  the  pistons  have  convex  heads 
and  where  particles  of  carbon  are  deposited  on 
the  piston  heads.  A  more  accurate  method  is 
to  make  use  of  a  timing  disk  attached  to  the 

[45] 


ELEMENTS  OF  AVIATION  ENGINES 

crank  shaft  near  the  propeller  hub.  The  cir- 
cumference of  this  disk  is  divided  into  degrees 
with  the  points  for  opening  and  closing  of  each 
valve  plainly  marked  upon  it.  If  the  disk  is 
placed  accurately  upon  the  crank  shaft  it  fur- 
nishes an  excellent  means  of  timing  the  valves, 
because  no  linear  measurements  need  be  taken. 
Since  the  angle  of  a  crank  throw  must  be  used 
when  working  with  a  timing  disk,  this  is  called 
the  angular  method  of  timing  valves. 


[46] 


CHAPTER  V 
CARBURETION 

IN  ORDER  that  gasoline  may  be  mixed  with 
the  right  amount  of  air  to  form  an  explosive 
mixture  within  the  cylinders,  it  is  necessary  to 
make  use  of  a  device  known  as  a  carburetor. 
A  great  deal  of  attention  has  been  devoted  to 
the  designing  of  carburetors,  for  it  can  be 
readily  seen  that  the  fuel  consumption  of  an 
engine  will  be  governed  largely  by  the  perform- 
ance of  the  carburetor.  Also  of  late  much 
attention  has  been  given  to  the  carburetion  of 
lower  grade  fuels,  so  the  subject  of  carburetors 
is  becoming  a  broad  field. 

A  suitable  mixture  for  an  aviation  engine  is 
one  pound  of  gasoline  to  fifteen  pounds  of  air. 
A  richer  mixture  would  be  one  having  more 
gasoline,  while  one  having  more  air  would  be 
a  leaner  mixture.  It  has  been  found  that  the 
most  practical  way  to  obtain  this  mixture  is  to 
spray  the  gasoline  into  the  air,  and  this  is  best 
accomplished  by  making  use  of  a  jet  attached 
to  a  reservoir  and  lessening  the  atmospheric 
pressure  about  the  jet.  If  the  level  of  gasoline 
in  the  reservoir  is  slightly  lower  than  the  tip 

[47] 


ELEMENTS  OF  AVIATION  ENGINES 

of  the  jet  and  the  jet  is  located  in  an  air-sup- 
plied chamber  having  a  connection  with  the 
inlet  valves,  the  downward  motion  of  the  pis- 
tons will  result  in  less  pressure  being  exerted 
upon  the  gasoline  in  the  jet  than  that  in  the 
reservoir,  where  atmospheric  pressure  is  ex- 
erted. Gasoline  in  this  way  will  be  made 
to  flow  from  the  jet,  and  since  considerable 
air  is  being  drawn  past  the  jet  it  will  tend 
to  form  a  spray  of  the  gasoline  that  is  being 
delivered. 

To  restrict  the  amount  of  gasoline  that  is 
supplied  to  the  float  chamber  which  in  turn 
has  a  great  deal  to  do  with  the  gasoline  deliv- 
ered by  the  jet,  the  float  with  which  the  float 
chamber  is  supplied,  actuates  a  pin  that  opens 
and  closes  the  main  supply  valve.  Upon  the 
top  of  the  float  rest  the  ends  of  two  pivoted 
arms  having  the  other  ends  in  contact  with 
the  needle  valve  stem.  As  gasoline  enters  the 
float  chamber  the  float  will  rise  causing  one  end 
of  the  arms  to  rise  and  the  other  end  to  exert 
a  downward  pressure  upon  the  needle  valve. 
The  result  will  be  to  seat  needle  valve  allowing 
no  more  gasoline  to  enter  until  some  has  been 
drawn  off  by  the  delivery  from  the  jet.  From 
this  it  can  readily  be  seen  that  the  float  cham- 

F481 


CARBURETION 

her  is  essentially  a  reservoir  supplied  with  an 
automatic  valve. 

The  space  around  the  jet  is  called  the  mixing 
chamber.  To  admit  the  necessary  air  an  open- 
ing is  located  somewhere  below  the  level  of  the 
jet  which  insures  all  of  the  air  passing  the  jet. 
As  a  means  of  diverting  the  air  nearer  the  tip 
of  the  jet  and  thus  securing  more  of  a  drawing 
effect,  the  space  around  the  jet  through  which 
the  air  passes  is  lessened  by  the  insertion  of  a 
choke  tube  or  a  venturi  as  it  is  often  called. 
Its  purpose  is  to  increase  the  velocity  of  air 
as  it  passes  by  the  jet  and  thus  increase  the 
suction  at  the  tip  of  the  jet.  To  regulate  the 
speed  of  the  engine  a  butterfly  valve  is  located 
just  a  little  distance  above  the  choke  tube. 
This  valve,  which  is  nothing  more  than  a  disk 
of  metal,  is  often  referred  to  as  the  throttle. 
When  it  is  opened  the  speed  of  the  engine  is 
increased  on  account  of  a  greater  volume  of 
gas  being  taken  by  the  engine.  As  it  is  brought 
toward  a  closed  position,  less  gas  will  be  sup- 
plied, and  the  result  is  to  decrease  the  speed  of 
the  engine.  Stop  screws  are  provided  to  pre- 
vent the  throttle  from  closing  completely,  for 
that  would  cause  the  engine  to  cease  running 
altogether. 

[49] 


ELEMENTS  OF  AVIATION  ENGINES 

So  far  the  most  elementary  type  of  carbu- 
retor has  been  discussed.  It  is  one  that  consists 
primarily  of  a  float  chamber  and  one  jet  in  a 
regularly  shaped  mixing  chamber.  This  is 
called  a  simple  jet  carburetor,  and  its  chief 
weakness  lies  in  the  fact  that  at  high  speeds  it 
will  deliver  a  richer  mixture  than  when  the 
engine  is  running  slowly;  the  reason  for  this 
being  that  as  the  speed  is  increased  the  suction 
is  greatly  increased,  which  means  more  gaso- 
line in  proportion  to  the  air  at  high  speeds  than 
at  low  speeds.  A  simple  jet  carburetor  ad- 
justed for  low  speeds  will  use  too  much  gaso- 
line at  high  speeds,  while  one  that  is  adjusted 
for  high  speeds  will  not  supply  enough  gasoline 
at  low  speeds.  Consequently  simple  jet  car- 
buretors are  not  satisfactory  for  aviation  en- 
gines. 

In  order  to  secure  the  right  mixture  at  both 
low  and  high  speeds,  several  modifications  of 
the  simple  jet  carburetor  have  been  used  with 
more  or  less  success.  One  way  is  to  have  the 
mixing  chamber  supplied  with  an  auxiliary  air 
valve  that  is  held  in  place  by  a  weak  spring. 
At  low  speeds  the  spring  holds  the  valve  closed, 
but  as  the  speed  is  increased  the  valve  is 
drawn  open  due  to  the  increase  in  suction. 

[50] 


THE  MILLER  AVIATION  CARBURETOR 


CARBURETION 

This  allows  more  air  to  enter  the  mixing  cham- 
ber at  high  speeds  causing  the  mixture  to  be- 
come slightly  leaner  and  thereby  securing 
approximately  the  same  mixture  at  high  speeds 
as  at  low  speeds.  Another  way  is  to  have  the 
opening  in  the  jet  supplied  with  a  metering  pin 
which  is  nothing  more  than  a  slender  pin 
tapered  to  a  point  that  extends  within  the  jet. 
As  the  throttle  is  qpened,  the  metering  pin  is 
withdrawn  much  more  slowly  proportionately 
than  the  throttle  is  turned.  This  will  allow  a 
slightly  greater  amount  of  gasoline  to  issue 
from  the  jet  at  high  speeds  than  at  low  speeds, 
but  if  arranged  correctly  the  increase  in  gaso- 
line will  be  in  proportion  to  the  increase  in  air. 
A  third  way  is  to  employ  several  jets  instead 
of  one,  and  by  using  a  rotary  throttle  uncover 
them  one  at  a  time  as  the  speed  is  increased, 
allowing  the  air  to  pass  by  more  than  one.  In 
this  manner  at  high  speeds  more  jets  are  ex- 
posed to  the  suction  than  at  low  speeds,  and 
likewise  the  size  of  the  air  opening  is  larger  at 
high  speeds  than  at  low  speeds.  A  uniform 
mixture  for  all  speeds  is  thus  secured.  A 
fourth  way  is  to  combine  two  small  jets  so  that 
one  will  deliver  more  and  more  gasoline  as  the 
speed  is  increased  and  the  other  will  deliver 

[51] 


ELEMENTS  OF  AVIATION  ENGINES 
only  a  limited  amount.  At  high  speeds  the  in- 
creased amount  of  gasoline  from  one  will  be 
just  enough  to  take  care  of  the  additional 
amount  needed.  At  low  speeds  both  jets  work 
harmoniously.  Such  departures  from  the  sim- 
ple jet  carburetor  are  spoken  of  as  speed  com- 
pensations. 

The  Zenith  carburetor  has  been  widely  used 
in  connection  with  aviation  engines,  and  for 
that  reason  it  will  be  well  to  become  familiar 
with  its  operation.  The  principle  used  is  that 
of  two  small  jets  with  one  having  only  a  lim- 
ited amount  of  gasoline  to  supply.  In  appear- 
ance it  closely  resembles  a  simple  jet  carbu- 
retor except  for  a  narrow  cylindrically-shaped 
well  between  the  float  chamber  and  the  mixing 
chamber.  Gasoline  is  supplied  from  the  float 
chamber  to  this  well  through  a  small  hole  in  a 
plug  that  forms  the  bottom  of  the  well.  The 
plug  is  called  the  compensator.  In  the  upper 
part  of  the  well  is  a  hole  which  allows  atmos- 
pheric pressure  to  be  exerted  upon  the  gasoline 
within.  One  jet  is  placed  within  the  other,  and 
the  inside  jet  is  that  one  connected  directly  to 
the  float  chamber.  Obviously  this  jet,  which  is 
known  as  the  main  jet,  will  act  the  same  as  one 
in  a  simple  jet  carburetor  causing  a  richer  mix- 

[52] 


SLOW  spfco  scacw 


A  HALF  SECTION  VIEW  OF  A  ZENITH  CARBURETOR 


.  . '  " 


CARBURETION 

ture  at  high  speeds  than  at  low  speeds.  The 
outside  jet  or  cap  jet,  as  it  is  called,  receives  its 
supply  of  gasoline  from  the  well,  and  since  the 
amount  of  gasoline  furnished  to  the  well  is 
limited  by  the  hole  in  the  compensator,  it  can 
be  seen  that  the  amount  of  gasoline  delivered 
by  the  cap  jet  is  restricted  to  that  amount  that 
will  flow  by  gravity  through  the  hole  in  the 
compensator.  At  low  speeds  both  jets  work 
normally,  but  as  the  speed  is  increased  the 
main  jet  will  furnish  more  and  more  gasoline 
while  there  will  be  a  tendency  to  draw  more 
gasoline  from  the  cap  jet  than  can  be  supplied 
by  the  hole  in  the  compensator.  The  result 
will  be  to  exhaust  the  supply  in  the  well  and 
use  instantly  that  which  is  fed  to  it.  Since 
there  is  an  air  hole  near  the  top  of  the  well  un- 
due suction  upon  the  compensator  will  be  pre- 
vented. It  should  be  noted  that  air  will  enter 
the  well  and  be  drawn  out  the  cap  jet  at  very 
high  speeds,  but  it  is  absolutely  wrong  to  re- 
gard the  air  hole  in  the  upper  part  of  well  as  an 
auxiliary  air  valve.  The  compensation  effect 
comes  from  the  fact  that  the  increased  amount 
of  gasoline  supplied  by  the  main  jet  is  enough 
to  make  up  for  that  which  is  not  supplied  by 
the  cap  jet. 

[53] 


ELEMENTS  OF  AVIATION  ENGINES 

At  idling  speed  very  little  air  is  drawn  in, 
and  this  is  not  sufficient  to  fully  overcome  the 
surface  tension  of  the  gasoline  in  the  jets.  By 
surface  tension  is  meant  the  force  that  tends 
to  resist  breaking  the  surface  of  the  column  of 
gasoline  in  the  jet  and  drawing  it  outward.  To 
insure  a  good  mixture  at  idling  speed  the 
Zenith  is  equipped  with  an  entirely  separate 
carburetor  that  supplies  its  gas  at  a  point 
where  the  air  passes  by  the  nearly  closed 
throttle,  and,  on  account  of  the  small  space, 
considerable  suction  is  developed  at  this  point. 
This  carburetor  gets  its  supply  of  gasoline 
through  a  tube  leading  down  near  the  bottom 
of  the  well.  The  tube  is  held  in  what  is  called 
the  priming  plug,  which  acts  as  a  cover  for  the 
well.  The  size  of  the  hole  in  the  priming  plug 
governs  the  amount  of  gasoline  fed  to  the 
idling  carburetor.  The  amount  of  air  that  is 
allowed  to  enter  the  mixing  chamber  of  the 
idling  carburetor  is  controlled  by  a  thumb 
screw  known  as  the  slow-speed  screw. 

To  facilitate  starting,  a  strangler  valve  is 
placed  in  the  air  inlet.  If  it  is  brought  toward  a 
closed  position  a  greatly  increased  amount  of 
gasoline  will  be  drawn  from  the  jets,  and  from 
this  increased  amount  the  more  readily  vola- 

[54] 


CARBURETION 

tile  parts  will  go  to  furnish  a  combustible  mix- 
ture. The  strangler  is  used  only  in  starting. 

The  variables  are  those  parts  affecting  the 
mixture  which  can  be  replaced  by  similar  ones 
having  different  dimensions.  In  naming  them 
they  are  generally  given  in  a  regular  order  be- 
ginning with  the  choke  tube,  then  the  main  jet, 
the  compensator,  and  finally  the  priming  plug. 
The  cap  jet  is  not  a  variable.  Zenith  settings 
comprise  the  internal  diameter  of  the  variables. 
The  choke  tube  is  measured  in  millimeters  and 
the  other  three  in  hundredths  of  a  millimeter. 
The  size  of  a  carburetor  is  the  diameter  in 
inches  of  its  connection  with  the  manifold. 

The  adjustments  on  the  Zenith  are  the 
throttle  stop  screw,  which  governs  the  suction 
upon  the  idling  carburetor,  the  slow-speed 
screw,  to  adjust  the  priming  plug's  delivery, 
and  adjusting  the  level  of  gasoline  in  the  float 
chamber  by  changing  the  position  of  the 
needle  valve  seat.  This  is  accomplished  by 
adding  washers  under  the  seat  if  the  level  is  to 
be  lowered  or  by  withdrawing  washers  if  the 
level  is  to  be  raised. 

As  a  plane  goes  from  a  low  altitude  to  a 
higher  one  the  effect  upon  the  carburetor  will 
be  to  furnish  a  richer  mixture,  since  the  same 

[55] 


ELEMENTS  OF  AVIATION  ENGINES 

volume  of  air  will  be  used,  yet  its  weight  will 
be  appreciably  decreased.  One  way  of  com- 
pensating for  altitude  is  to  have  an  air  valve 
located  in  the  manifold  that  can  be  opened  by 
the  pilot  as  necessity  calls  for.  The  effect  of 
opening  this  valve  will  be  to  allow  a  little  air  to 
be  added  to  the  rich  mixture.  Another  method 
is  to  decrease  the  pressure  upon  the  level  of 
gasoline  in  the  float  chamber  by  opening  a 
valve  in  a  tube  leading  from  the  float  chamber 
cover  to  the  manifold.  The  reduced  pressure 
within  the  manifold  in  this  way  is  used  to 
slightly  reduce  the  atmospheric  pressure  upon 
the  gasoline  in  the  float  chamber.  The  effect 
will  be  to  cause  less  difference  in  pressure  be- 
tween the  gasoline  in  the  float  chamber  and 
the  gasoline  in  the  jets,  resulting  in  a  decrease 
in  the  amount  of  gasoline  delivered  at  the  jets. 


[56] 


CHAPTER  VI 
IGNITION 

THE  ELECTRIC  spark,  which  is  the  only  sat- 
isfactory means  of  igniting  a  charge  of  gas 
in  an  internal  combustion  engine,  is  furnished 
by  current  coming  from  batteries  or  a  mag- 
neto. In  a  battery  electricity  is  generated  by 
chemical  action  while  the  magneto  is  a  me- 
chanical means  of  generating  electricity.  Al- 
though the  care  of  a  battery  is  important,  there 
is  no  call  for  an  extensive  knowledge  of  bat- 
tery construction  to  keep  it  in  good  condition. 
With  a  magneto,  however,  there  are  many 
moving  parts  which  need  attention  and  fre- 
quently adjustments  are  necessary,  so  it  seems 
advisable  to  take  up  the  magneto  rather  fully. 
To  start  with  the  fundamentals  of  electricity 
it  will  be  remembered  that  if  a  coil  of  wire  is 
revolved  between  the  poles  of  a  horseshoe  mag- 
net so  that  it  cuts  the  lines  of  magnetic  force 
there  will  be  a  current  generated  in  the  wire 
that  goes  to  make  up  the  coil.  Furthermore,  if 
this  coil  is  wound  about  a  piece  of  soft  iron 
known  as  a  core,  and  the  core  revolved  be- 
tween the  two  magnetic  poles  so  that  magnet- 

[57] 


ELEMENTS  OF  AVIATION  ENGINES 

ism  passes  through  the  core  one  instant  and 
not  the  next,  then  more  electricity  will  be  gen- 
erated. The  core  offers  an  easy  path  for  the 
magnetism.  Soft  iron  is  used  for  the  core  be- 
cause it  can  very  quickly  become  magnetized, 
and  what  is  just  as  important  it  will  quickly 
give  up  its  magnetism.  By  revolving  such  a 
core  between  the  poles  of  a  horseshoe  magnet, 
it  will  amount  to  successively  plunging  a  mag- 
net in  a  coil  and  rapidly  drawing  it  out  again. 
The  magnetic  lines  of  force  from  the  core, 
when  it  is  magnetized,  will  of  course  be  cut  by 
the  coil  which  accounts  for  the  current. 

As  the  core  is  revolved  between  the  two 
magnetic  poles,  which  are  distinguished  by 
calling  one  the  North  pole  and  the  other  the 
South  pole,  the  core  is  magnetized  when  in  a 
horizontal  position  almost  connecting  the  two 
poles  and  demagnetized  when  it  has  turned 
90  degrees  to  a  vertical  position.  Consequently 
in  one  complete  revolution  of  the  core  it  will 
be  magnetized  twice  and  demagnetized  twice. 
It  so  happens  that  a  little  more  current  is  gen- 
erated in  the  coil  when  the  core  loses  its  mag- 
netism than  when  it  receives  its  magnetism, 
which  means  that  maximum  current  is  ob- 
tained when  the  core  is  approximately  in  a 

[58] 


N 
L 


t 


POSITION  or  GORE 


POSITIOH  or  CORE. 

/S  /?/?0Af  EN , 


DIAGRAMS  TO  ILLUSTRATE  THE  LOCATION  OF  THE  CORE  IN  A 
SHUTTLE  TYPE  MAGNETO 


IGNITION 

vertical  position.  Since  it  is  in  this  position 
twice  during  one  complete  revolution,  it  fol- 
lows that  an  ordinary  magneto  furnishes  two 
sparks  per  revolution. 

So  far  the  core  has  been  considered  as  the 
revolving  part.  Identically  the  same  result  is 
obtained  when  the  core  is  held  stationary  and 
the  magnet  or  magnets  are  revolved.  To  turn 
the  magnets  is  inconvenient  on  account  of  their 
horseshoe  shape,  so  rotating  poles  are  often 
used  to  accomplish  the  same  result.  This  is 
referred  to  as  a  revolving  field.  In  the  first 
case  where  the  core  is  rotated,  an  armature 
made  up  of  the  core  and  the  shaft  that  carries 
it  is  used.  In  appearance  the  armature  has 
somewhat  of  a  resemblance  to  a  shuttle,  on 
account  of  the  windings  about  the  core.  For 
this  reason  the  type  of  magneto  using  the  re- 
volving core  and  coil  is  called  the  shuttle  type, 
while  the  one  in  which  the  magnetic  field  re- 
volves and  the  coil  remains  stationary  is  known 
as  the  inductor  type.  More  attention  will  be 
devoted  to  the  inductor  type  after  the  shuttle 
type  has  been  further  explained. 

The  current  required  to  jump  the  gap  be- 
tween the  points  of  a  spark  plug  under  high 
compression  is  much  greater  than  the  amount 

[59] 


ELEMENTS  OF  AVIATION  ENGINES 

supplied  by  a  single  coil  wound  about  a  core. 
In  order  to  have  a  self-contained  unit  it  is 
necessary  to  make  use  of  another  coil  wound 
about  the  first  consisting  of  much  finer  wire 
and  having  several  hundred  times  as  many 
turns  as  in  the  first  coil.  The  coil  wound  near- 
est the  core  is  called  the  primary  coil,  while  the 
outside  one  is  the  secondary  coil.  Now  if  we 
have  some  automatic  device  to  break  the  path 
of  the  current  from  the  primary  coil  at  the 
same  time  that  the  core  loses  its  magnetic 
charge  a  high  tension  current  will  be  induced 
into  the  secondary  coil  and  will  be  suitable  to 
conduct  to  the  spark  plugs.  The  principle  is 
that  of  a  transformer. 

On  the  shuttle  type  magneto  a  breaker  me- 
chanism through  which  the  primary  current 
passes  is  held  on  one  end  of  the  armature, 
causing  it  to  be  revolved  at  exactly  the  same 
speed  as  the  armature  is  turning.  Cams  on  the 
breaker  housing  force  the  breaker  points  to 
separate  for  an  instant,  at  the  same  time  that 
the  core  loses  its  magnetism.  All  that  is  neces- 
sary to  do  in  order  to  stop  the  magneto  from 
delivering  current  and  in  turn  stop  the  engine 
is  to  close  a  switch  on  a  line  that  connects  the 
two  breaker  points.  This  will  short  circuit 

[60] 


IGNITION 

the  primary,  destroying  the  effectiveness  of  the 
breaker  points  and  prevent  the  primary  from 
inducing  any  current  into  the  secondary  coil. 

To  advance  or  retard  the  spark,  the  position 
of  the  breaker  cams  is  changed.  This  affects 
the  time  that  the  primary  circuit  is  broken. 
Moving  the  cams  with  the  direction  of  rota- 
tion retards  the  spark,  while  to  advance  the 
spark  the  cams  are  moved  against  the  direc- 
tion of  rotation.  This  brings  us  to  one  diffi- 
culty with  the  shuttle  type  magneto.  In  order 
to  get  the  maximum  current  the  primary  circuit 
should  be  broken  as  the  core  loses  its  magnetic 
charge.  If  the  spark  is  retarded,  however,  the 
primary  is  broken  a  little  later  than  the  mag- 
netic lines  of  force  are  broken,  which  results  in  a 
weaker  spark.  The  effect  is  frequently  to  hind- 
er starting  as  it  is  necessary  to  retard  the  spark 
to  prevent  injuring  the  one  who  is  cranking. 

In  the  primary  circuit  a  condenser  is  placed 
in  multiple  with  the  breaker  points.  It  con- 
sists of  alternate  sheets  of  a  conductor  and  a 
non-conductor  such  as  tinfoil  and  mica.  Half 
of  the  sheets  of  the  conductor  are  attached  to 
one  terminal,  and  the  other  half  are  attached 
to  the  second  terminal.  This  provides  a  place 
for  the  current  to  go  momentarily  after  the 

[611 


ELEMENTS  OF  AVIATION  ENGINES 
breaker  points  have  separated.  If  a  condenser 
were  not  used  there  would  be  a  tendency  for 
the  current  to  continue  flowing  for  an  instant 
through  the  air  between  the  separating  points, 
which  would  result  in  arcing  and  pitting  the 
points.  Right  here  it  should  be  noted  that  the 
breaker  points  are  of  platinum  and  should  not 
separate  more  than  .020  inch.  Since  the  con- 
denser prevents  arcing  it  also  serves  to  make 
the  break  in  the  primary  circuit  occur  more 
quickly,  which  means  that  more  voltage  will 
be  induced  into  the  secondary  coil. 

As  a  means  of  conducting  the  secondary  cur- 
rent to  the  right  spark  plug  at  the  right  time  a 
distributor  is  used.  It  consists  of  as  many  seg- 
ments as  the  number  of  spark  plugs  that  the 
magneto  supplies.  A  distributor  arm  with  a 
carbon  brush  directly  connected  with  the  sec- 
ondary coil  turns  about  upon  the  distributor 
plate  conducting  secondary  current  to  each 
segment  in  turn.  With  the  spark  fully  ad- 
vanced, the  distributor  arm  should  just  be 
entering  a  segment  every  time  the  breaker 
points  separate.  For  convenience  the  primary 
and  secondary  circuits  are  both  grounded. 
Should  the  secondary  circuit  be  left  open,  as 
would  be  the  case  if  a  wire  were  not  attached 

162] 


WIRING  DIAGRAM  OF  A  MAGNETO  SYSTEM 


IGNITION 

to  a  spark  plug,  the  result  might  be  that  the 
high  pressure  of  the  secondary  would  cause  a 
short  circuit  between  the  two  coils.  To  avoid 
such  a  happening  a  safety  gap  is  provided  in 
the  secondary  circuit.  Its  points  are  generally 
three-eighths  of  an  inch  apart  insuring  no  in- 
terference with  sparking  at  the  plugs. 

The  electrical  pressure  is  expressed  in  volts. 
The  flow  is  expressed  in  amperes.  One  volt 
times  one  ampere  is  equivalent  to  one  watt, 
which  is  nothing  more  than  a  unit  of  work, 
being  1  /764  part  of  a  horse-power.  The  wattage 
of  an  ordinary  magneto  is  about  twenty.  The 
voltage  in  the  primary  circuit  is  from  six  to  ten 
volts,  while  that  in  the  secondary  is  about  ten 
thousand.  The  amperage  of  the  primary  is 
limited  to  only  a  few  amperes,  yet  that  of  the 
secondary  is  infinitely  less,  being  only  the 
slightest  fraction  of  an  ampere,  for  it  should  be 
remembered  that  when  a  current  of  higher 
voltage  is  obtained  by  induction  the  gain  in 
the  number  of  volts  will  be  accompanied  by  a 
loss  in  the  number  of  amperes.  Upon  the  speed 
of  the  armature  and  the  number  of  windings 
depends  the  voltage  of  a  magneto.  The  num- 
ber of  amperes  is  dependent  upon  the  strength 
of  the  magnets. 

[631 


ELEMENTS  OF  AVIATION  ENGINES 

An  ordinary  magneto  can  deliver  but  two 
sparks  per  revolution,  so  the  speed  of  the  arma- 
ture is  governed  by  the  number  of  explosions 
in  the  engine  during  one  complete  cycle.  A 
four-cylinder  engine  will  fire  four  times  in  two 
revolutions  or  two  times  in  one  revolution. 
Since  two  sparks  will  then  be  necessary  for 
every  revolution  of  the  crank  shaft,  it  follows 
that  the  armature  should  turn  at  engine  speed. 
In  an  eight-cylinder  engine  there  will  be  four 
explosions  per  revolution,  so  the  armature  will 
have  to  turn  at  twice  engine  speed  to  give  the 
four  sparks  at  the  right  time.  The  speed  of  an 
armature  on  a  magneto  supplying  twelve 
cylinders  would  be  three  times  engine  speed. 
A  convenient  means  of  determining  this  rela- 
tive speed  is  to  divide  the  number  of  cylinders 
by  four.  The  distributor  arm  turns  at  cam- 
shaft speed  owing  to  the  fact  that  each  cylin- 
der requires  one  spark  in  two  revolutions  of  the 
engine. 

The  Dixie  magneto,  which  is  a  good  example 
of  the  inductor  type,  has  been  widely  used  and 
deserves  consideration.  In  it  the  magnets  are 
turned  at  right  angles  to  the  position  that  they 
occupy  in  the  Bosch  and  Berling,  which  are 
representatives  of  the  shuttle  type.  A  shaft 

164] 


DIAGRAM  TO  ILLUSTRATE  THE  PRINCIPLE  OF  REVOLVING 
POLES  ON  THE  DIXIE  MAGNETO 


IGNITION 

carrying  two  shoes  or  pole  extensions  separated 
by  a  bronze  block  is  placed  in  line  with  the  two 
poles  of  the  magnets.  This  shaft  having  no 
windings  upon  it  is  not  called  an  armature, 
but  is  known  as  a  rotor.  As  the  rotor  is  re- 
volved the  shoes,  each  being  in  contact  with 
one  pole  and  being  separated  by  the  non-mag- 
netic bronze,  will  always  have  their  respective 
magnetic  charges,  and  the  effect  will  be  much 
the  same  as  though  the  magnets  themselves 
were  revolved.  Were  it  not  for  the  bronze  be- 
tween the  two  shoes  there  would  be  a  direct 
flow  of  magnetism  through  the  rotor  between 
the  two  poles,  and  the  shoes  would  then  be 
useless. 

At  right  angles  to  the  rotor  is  placed  the  core 
carrying  the  primary  and  secondary  coils.  It 
is  located  in  the  space  between  the  rotor  and 
the  top  of  the  magnets.  Extending  downward 
from  both  ends  of  the  core  are  two  bars  of  soft 
iron  known  as  field  pieces,  and  it  is  between 
these  two  field  pieces  that  the  shoes  revolve. 
When  the  shoes  are  in  a  horizontal  position, 
magnetism  will  pass  from  one  shoe  into  the 
nearest  field  piece,  then  through  the  core,  into 
the  other  field  piece,  and  thence  into  the  oppo- 
site shoe.  When  the  shoes  move  to  a  vertical 

[65] 


ELEMENTS  OF  AVIATION  ENGINES 

position  the  core  will  receive  no  magnetism, 
but  in  moving  another  90  degrees  the  shoes 
will  come  again  to  a  horizontal  position,  and 
magnetism  will  pass  through  the  core  in  a  re- 
'  versed  direction.  Thus  the  core  will  be  mag- 
netically charged  one  instant  and  not  the  next, 
resulting  in  the  generating  of  electricity. 

The  breaker  assembly  does  not  revolve  on 
the  end  of  the  rotor,  but  is  worked  by  cams  on 
the  end  of  the  rotor  shaft.  To  advance  or  re- 
tard the  spark  it  is  thus  possible  to  move  the 
whole  breaker  assembly  instead  of  changing 
the  position  of  fixed  cams,  as  is  done  on  the 
shuttle  type.  Since  the  coils  and  core  do  not 
revolve,  it  is  also  possible  to  change  the  posi- 
tion of  the  core  and  field  pieces  with  the 
changing  of  the  position  of  the  breakers.  The 
result  is  to  break  the  magnetism  in  the  core 
with  the  breaking  of  the  primary  circuit  even 
though  the  spark  is  fully  retarded.  This  in- 
sures the  same  intensity  of  spark  when  crank- 
ing the  engine  as  is  obtained  at  top  speed  with 
the  spark  fully  advanced. 

A  special  type  of  Dixie  magneto  is  one  hav- 
ing four  shoes  instead  of  two.  There  are  two 
opposite  North  shoes  and  two  opposite  South 
shoes.  The  two  field  pieces  leading  to  the  core 

[661 


nnnnnnnnn  n  nn  nn  n  n  fin 


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/l(/l/l/f/Ul/UI/UVUVUUUV 


DIAGRAM  TO  ILLUSTRATE  POSITION  OF  ROTOR  IN  THE  DIXIE 
MAGNETO  WHEN  THE  CORE  IS  MAGNETIZED 


/  If  I    II   /f    //    II   II  /I/I    I 

'III.iI.un 

VUUUUUUl/  V 


u 


DIAGRAM  TO  ILLUSTRATE  POSITION  OF  ROTOR  IN  THE  DIXIE 
MAGNETO  WHEN  THE  CORE  IS  DEMAGNETIZED 


IGNITION 

are  shortened  so  that  their  ends  will  be  well 
above  the  center  of  the  rotor  to  allow  unlike 
shoes  to  connect  the  two  ends.  Since  the  oppo- 
site shoes  have  the  same  polarity,  it  would  not 
do  to  have  the  ends  of  the  field  pieces  in  line 
with  opposite  shoes.  The  advantage  of  this 
type  of  Dixie  is  that  four  sparks  can  be  secured 
during  one  revolution  of  the  rotor,  which  per- 
mits a  much  slower  running  magneto  on  an 
engine  having  a  large  number  of  cylinders. 

The  magnets  themselves  deserve  attention. 
They  are  made  of  hard  steel  in  order  to  retain 
their  magnetism  as  long  as  possible.  To  re- 
charge a  magnet  it  is  simply  necessary  to  wind 
it  with  insulated  wire  and  pass  direct  current 
through  the  wire.  When  dismounted  from  the 
magneto  it  is  necessary  to  provide  a  path  for 
the  magnetism  between  the  two  poles.  A  strip 
of  steel  will  answer  for  a  keeper,  or  the  unlike 
poles  of  two  magnets  may  be  placed  together, 
insuring  a  perfect  magnetic  circuit. 

In  timing  a  magneto  to  an  engine  it  is  best 
to  start  by  selecting  a  cylinder  and  placing  the 
piston  in  that  cylinder  in  the  right  position  on 
the  compression  stroke  for  the  spark  to  occur. 
Then  turn  the  distributor  arm  so  that  it  is 
about  to  enter  the  segment  which  has  connec- 

[67] 


ELEMENTS  OF  AVIATION  ENGINES 

tion  with  the  spark  plug  in  the  selected  cylin- 
der. Next,  after  making  sure  the  spark  lever 
is  fully  advanced,  turn  the  magneto  in  its  right 
direction  until  the  breaker  points  have  just 
separated.  The  distributor  arm  should  now  be 
upon  the  foremost  part  of  the  segment  to  in- 
sure that  it  will  still  be  in  contact  with  the  seg- 
ment when  the  spark  lever  is  retarded.  The 
remaining  step  is  to  connect  the  driving  shaft 
with  the  armature  or  rotor  as  the  case  may  be. 
Upon  those  engines  having  double  ignition  to 
procure  a  greater  factor  of  safety  and  to  reduce 
the  time  to  fully  explode  the  charge,  the  two 
magnetos  must  furnish  their  sparks  at  the  same 
time  or  be  synchronized  as  it  is  technically 
called.  To  accomplish  this  the  first  magneto  is 
timed  to  the  engine  and  the  second  magneto  is 
timed  to  the  first.  In  this  way  the  breaker 
points  on  each  one  can  be  made  to  separate  at 
the  same  instant. 

With  a  battery  system  either  a  vibrating  or 
a  non-vibrating  coil  may  be  used.  Vibrating 
coils  will  give  a  rapid  succession  of  sparks  at 
the  spark  plugs.  The  primary  circuit  is  made 
to  pass  through  the  vibrator  and  the  magnet- 
ism in  core  is  allowed  to  separate  the  vibrator 
points  which  breaks  the  primary  circuit  and 

[68] 


I WWW <•*-! 


DIAGRAM  OF  A  BATTERY  SYSTEM  OF  IGNITION 
WITH  A  NON  VIBRATING  COIL 


IGNITION 

demagnetizes  the  core.  The  contact  is  then 
made  again  by  the  vibrator  springing  back  and 
the  operation  is  repeated.  When  a  non- vibrat- 
ing coil  is  used  there  must  be  mechanically- 
operated  breakers  to  break  the  primary  circuit, 
very  similar  to  those  used  on  magnetos. 

The  wiring  diagram  for  a  battery  system 
using  mechanically-operated  breakers  is  simi- 
lar to  a  magneto  wiring  diagram,  except  that 
the  switch  is  not  placed  in  the  same  position. 
In  a  battery  system  the  switch  is  placed  in 
series  in  the  primary  circuit,  and  by  opening 
the  switch  the  engine  is  stopped.  Sometimes 
two  breakers  are  used  instead  of  a  single  one. 
The  two  are  then  wired  in  multiple  and  are 
made  to  break  at  the  same  time,  thereby  in- 
suring uninterrupted  flight  in  case  one  refuses 
to  close.  A  small  resistance  coil  of  iron  wire 
is  often  placed  in  the  primary  circuit  with  a 
view  to  saving  the  battery  during  slow  run- 
ning, or  in  case  the  switch  is  left  closed  when 
the  engine  is  not  running.  Ordinarily  when 
the  engine  stops  the  breaker  points  are  to- 
gether, which,  with  a  closed  switch,  affords  a 
direct  path  for  the  current  to  pass  from  one 
pole  of  the  battery  to  the  other.  The  iron  coil 
will  then  be  heated  with  the  result  that  less 

[69] 


ELEMENTS  OF  AVIATION  ENGINES 

current  can  pass  through  the  heated  iron  wire 
than  when  it  was  cold.  In  this  way  a  battery 
will  not  be  exhausted  so  readily.  A  coil  that 
serves  this  purpose  is  called  a  ballast  coil. 


70] 


CHAPTER  VII 
LUBRICATION 

THE  PURPOSE  of  lubrication  is  to  reduce 
friction.  Even  though  two  pieces  of  metal 
that  move  one  upon  the  other  may  have  their 
surfaces  highly  polished  and  appear  perfectly 
smooth,  it  will  be  noticed  upon  examination 
with  a  microscope  that  the  surfaces  are  very 
irregular.  In  other  words  all  sliding  surfaces, 
no  matter  how  carefully  they  may  be  finished, 
are  known  to  consist  of  minute  projections  and 
depressions.  Consequently  the  projections  and 
hollows  on  the  contact  faces  tend  to  interlock 
and  resist  a  sliding  motion.  From  this  it  can 
readily  be  seen  that  friction  is  nothing  more 
than  the  force  which  resists  the  relative  motion 
of  one  body  in  contact  with  another  body. 
Excessive  friction  results  in  the  development 
of  heat. 

As  a  means  of  minimizing  friction,  oil  is  in- 
troduced between  the  contact  surfaces.  The 
oil  will  first  fill  the  depressions  and  finally 
form  a  film  between  the  two  surfaces,  separat- 
ing them  sufficiently  to  prevent  the  projections 
on  one  surface  from  interlocking  with  the  de- 

[71] 


ELEMENTS  OF  AVIATION  ENGINES 

pressions  on  the  other.  This  is  referred  to  as 
the  theory  of  lubrication.  Perfect  lubrication 
is  greatly  to  be  desired  for  it  eliminates  wear, 
and  by  reducing  the  power  required  to  turn  the 
engine  it  adds  to  the  efficiency  of  an  engine. 

After  realizing  the  necessity  for  oil  the  next 
step  is  to  ascertain  what  properties  an  oil  must 
have  in  order  that  it  may  be  suitable  for  avia- 
tion engines.  In  testing  an  oil  it  is  customary  to 
determine  the  gravity,  viscosity,  flash  point,  fire 
point,  and  whether  or  not  it  has  acid  properties. 

The  gravity  of  an  oil  has  in  reality  no  effect 
upon  its  lubricating  merits,  as  there  is  con- 
siderable variation  in  the  gravity  of  high  grade 
oils.  However,  it  is  usually  determined  and 
used  principally  in  checking  current  deliveries 
of  a  certain  brand.  The  specific  gravity  is  the 
ratio  between  its  weight  and  the  weight  of  an 
equal  volume  of  water.  In  the  oil  trade,  though, 
it  is  customary  to  use  the  Baume  gravity  scale 
in  which  the  gravity  of  water  is  10  at  60°  F. 
The  lighter  the  oil  is  in  body,  the  higher  will 
be  the  Baume  reading.  Hydrometers  gradu- 
ated for  either  specific  gravity  or  Baume  are 
used  to  measure  the  gravity  of  an  oil.  The  fol- 
lowing formula  will  serve  to  convert  one  scale 
in  another: 

[72] 


LUBRICATION 

Specific  gravity =; 


130+ Baume  reading* 

Viscosity  is  the  technical  name  for  what  is 
popularly  called  "body."  To  express  it  more 
specifically  it  is  the  fluidity  of  an  oil.  To  ob- 
tain the  viscosity  the  oil  is  put  into  a  cup  sur- 
rounded by  water  at  about  212°  F.  When  the 
oil  has  reached  this  temperature,  a  plug  of 
specific  size  in  the  bottom  of  the  cup  is  re- 
moved allowing  60  c.c.  of  the  hot  oil  to  run  out 
into  a  marked  flask.  The  number  of  seconds 
required  to  draw  the  60  c.c.  is  reported  as  the 
viscosity  of  the  oil.  Good  cylinder  oil  will  have 
a  viscosity  of  about  75  seconds. 

The  flash  point  is  the  lowest  temperature  at 
which  the  oil  will  ignite  but  not  continue  to 
burn.  If  the  flash  point  is  too  low,  the  oil  will 
not  remain  on  the  cylinder  walls  and  bearings 
when  the  normal  heat  is  developed,  leaving  the 
friction  surfaces  without  lubrication.  It  is  well 
to  use  oil  having  a  flash  point  above  325°  F. 

The  fire  point  is  the  temperature  at  which 
the  ignited  vapor  from  the  oil  will  continue  to 
burn.  This  temperature,  which  ranges  be- 
tween 45°  and  75°  F  above  the  flash  point,  is 
not  of  much  consequence  from  our  standpoint 
as  it  is  always  beyond  the  point  where  the  oil 
will  cease  to  be  useful. 

[73] 


ELEMENTS  OF  AVIATION  ENGINES 

Certain  mineral  oils  are  treated  with  sul- 
phuric acid  during  the  process  of  refinement. 
To  protect  the  highly  polished  bearing  sur- 
faces it  may  be  necessary  to  ascertain  if  any 
acid  has  remained  in  the  oil.  A  simple  way  of 
testing  is  to  wash  a  sample  of  the  oil  with 
warm  water  and  test  the  water  with  litmus 
paper.  The  presence  of  any  acid  will  result  in 
the  paper  being  turned  pink. 

Lubricating  oil  we  are  accustomed  to  think 
of  as  being  only  mineral  oil.  With  the  develop- 
ment of  aviation  engines,  castor  oil,  which  is  a 
vegetable  oil,  has  received  considerable  atten- 
tion. This  oil,  which  has  a  gravity  of  96° 
Baume  and  a  flash  point  a  little  higher  than 
most  mineral  oils,  will  thicken  to  a  marked  de- 
gree upon  standing.  When  heated  it  will 
readily  oxidize  and  exhibit  acid  properties, 
rendering  it  of  little  use  in  engines  where  the 
oil  is  used  over  and  over  again.  Its  universal 
use  in  rotary  engines  is  due  to  the  fact  that  it 
will  not  unite  with  gasoline.  In  these  engines 
the  crank  case  is  filled  with  gasoline  vapor  which 
tends  to  wash  off  any  mineral  oil  that  is  supplied 
to  the  bearings.  Hence  castor  oil  is  resorted 
to,  and  as  long  as  the  oil  is  used  but  once  in 
rotary  engines,  it  serves  very  well  as  a  lubricant. 

[74] 


LUBRICATION 

Very  few  engines  have  the  same  system  of 
lubrication.  The  oil  supply  is  generally  carried 
in  the  lower  half  of  the  crank  case  which  is 
called  the  sump.  Frequently  auxiliary  tanks 
having  connection  with  the  sump  are  used. 
The  splash  system  of  oiling  is  not  suitable  for 
aviation  engines,  so  it  is  possible  to  make  use 
of  what  is  called  a  dry  sump.  If  no  provision 
is  made  to  retain  the  oil  in  the  sump  when  an 
engine  is  momentarily  inverted  there  is  great 
danger  of  the  oil  rushing  into  the  cylinder  and 
piston  cavities.  However,  if  the  lower  half  of 
the  crank  case  has  a  false  bottom  the  oil  may 
be  carried  in  the  compartment  thus  formed 
with  no  danger  of  it  rushing  out.  Another  way 
to  obtain  a  dry  sump  is  to  collect  the  returning 
oil  from  the  bearings  in  a  trap  at  the  bottom  of 
the  crank  case  and  pump  it  away  to  a  tank 
where  the  main  supply  is  located. 

A  gear  pump  is  generally  used  to  force  the 
oil  to  the  bearings.  Its  construction  is  remark- 
ably simple  as  it  consists  of  two  rotating  gears 
in  a  closely-fitted  housing.  Oil  is  caught  in  the 
spaces  between  the  successive  teeth  of  each 
gear  and  carried  around  to  the  discharge  of  the 
pump.  Plunger  pumps  and  vane  pumps  are 
also  used.  The  Hispano-Suiza  engine  makes 

[751 


ELEMENTS  OF  AVIATION  ENGINES 

use  of  the  vane  pump  to  develop  a  high  oil 
pressure. 

A  pressure  relief  valve  is  usually  placed  on 
the  oil  line  very  near  the  pump.  Such  a  valve 
consists  essentially  of  a  poppet  valve  which 
opens  outwardly  and  which  is  held  in  place  by 
a  spring  fitted  with  a  cap  screw.  In  case  the 
line  were  to  become  obstructed,  the  valve  will 
open  relieving  the  pressure  and  permitting  the 
oil  to  return  to  the  sump.  By  changing 
the  tension  of  the  spring  with  the  cap  screw  the 
oil  pressure  may  be  regulated.  A  sight  gage 
in  the  cock  pit  is  used  to  indicate  the  pressure. 
It  should  be  connected  to  the  main  oil  line  at 
a  point  not  far  distant  from  the  pump.  When 
plunger  pumps  are  used,  pulsators  are  often 
employed  to  show  the  operation  of  the  pumps. 
A  pulsator  consists  of  a  glass  dome  in  which  a 
quantity  of  air  is  compressed  by  the  entrance 
of  some  oil  from  the  main  oil  line.  The  im- 
pulses of  the  plunger  in  the  pump  will  give  rise 
to  a  throbbing  motion  of  the  surface  of  oil  in 
the  glass  dome. 

While  the  oiling  system  in  the  Curtiss  OX 
can  not  be  regarded  as  representing  the  way 
all  aviation  engines  are  oiled,  it  may  be  well  to 
describe  it  and  point  out  its  peculiarities.  In 

[76] 


GEAR  PUMP 


DIAGRAM  TO  ILLUSTRATE  THE  OPERATION  OF  A  VANE  PUMP 


LUBRICATION 

this  engine  the  oil  is  carried  in  the  sump  where 
it  is  covered  with  splash  pans.  Toward  the 
propeller  end  of  the  sump  is  located  a  gear 
pump  which  forces  the  oil  under  a  pressure  of 
about  fifty  pounds  to  the  propeller  end  of  the 
hollow  cam  shaft.  At  each  of  the  five  cam- 
shaft bearings  a  hole  is  drilled  allowing  oil  to 
escape  and  oil  these  bearings.  Directly  be- 
neath each  cam-shaft  bearing  is  a  crank-shaft 
bearing.  The  partitions  in  the  crank  case  or 
webs  as  they  are  called,  which  connect  the 
cam-shaft  bearings  with  the  crank-shaft  bear- 
ings, are  drilled.  In  this  way  oil  is  supplied  to 
grooves  in  the  cam-shaft  bearings,  whence  it 
is  forced  down  the  holes  in  the  webs  to  the 
crank-shaft  bearings.  Radially-drilled  holes  in 
the  hollow  crank  shaft  at  each  of  the  five  main 
bearings  allow  oil  to  pass  once  in  each  revolu- 
tion from  the  holes  in  the  crank  case  webs  into 
the  crank  shaft.  Centrifugal  force  carries  it 
out  the  crank-shaft  throws.  Each  crank  pin 
has  two  holes  drilled  in  that  part  of  the  pin 
directly  away  from  the  center  of  rotation.  In 
this  way  centrifugal  force  carries  oil  out  of  the 
crank  pins  to  oil  the  two  lower  connecting-rod 
bearings  upon  each  crank  pin.  The  seepage 
from  these  bearings  develops  a  spray  of  oil 

[77] 


ELEMENTS  OF  AVIATION  ENGINES 

within  the  crank  case.  As  a  piston  moves  up- 
ward, exposing  part  of  the  cylinder  wall,  the 
spray  comes  in  contact  with  the  wall  and  oils 
it.  Also  the  spray  is  made  use  of  to  oil  the 
wrist-pin  bearings  in  the  piston  bosses  by 
allowing  it  to  enter  holes  drilled  in  the  bosses. 
Since  the  cam  shaft  is  located  within  the  crank 
case  in  this  engine  the  cam  surfaces  will  be 
oiled  by  the  spray.  The  magneto  gear  and  cam 
shaft  gear  are  oiled  by  spray  from  a  hole  in  the 
retaining  screw  of  the  cam-shaft  gear.  The 
thrust  bearing  receives  what  little  oil  it  re- 
quires from  a  small  hole  in  the  crank  shaft  near 
this  bearing.  The  most  peculiar  feature  of  the 
system  is  that  the  cam  shaft  is  used  as  a  main 
distributing  line.  A  crank  shaft  hollow  through- 
out is  another  striking  feature. 

Some  aviation  engines  are  equipped  with 
cooling  devices  for  the  oil.  Their  object  is  to 
maintain  a  suitable  viscosity  and  also  a  slight 
cooling  effect  upon  the  bearings.  The  Hall- 
Scott  engine  makes  use  of  an  oil-cooling  jacket 
very  near  the  carburetor  on  the  intake  mani- 
fold. Owing  to  the  fact  that  vaporization  is 
accompanied  by  the  extraction  of  heat  from 
surrounding  bodies,  the  oil  is  cooled  by  the 
vaporizing  of  gasoline.  The  Thomas-Morse 

[78] 


LUBRICATION 

engine  passes  its  oil  through  a  coil  of  pipe 
known  as  an  oil  radiator  which  is  located  in  a 
sufficiently  cool  place.  An  auxiliary  oil  tank  is 
used  in  connection  with  the  Sturtevant  engine. 
By  passing  the  oil  to  and  from  the  tank  a  low- 
ering in  temperature  is  secured.  Another 
method  used  on  some  foreign  engines  is  to  pass 
air  tubes  through  the  sump,  and,  by  drawing 
air  through  these  tubes  to  the  carburetor,  a 
cooling  effect  upon  the  oil  in  the  sump  will  be 
gained. 


[79] 


CHAPTER  VIII 
COOLING 

A  RAPID  succession  of  explosions  within  a 
cylinder  would  soon  heat  its  interior  to 
redness  if  some  means  were  not  taken  to  con- 
duct the  heat  away.  Such  high  temperature 
would  burn  the  lubricating  oil  and  cause  the 
pistons  to  seize  and  the  bearings  to  burn  out. 
Excessive  heat  will  also  cause  irregularities  in 
the  combustion  chamber  to  become  so  highly 
heated  that  they  will  ignite  a  fresh  charge  of 
gas.  Obviously  some  of  the  heat  must  be  con- 
ducted away.  A  great  danger  comes  into 
account  at  this  point,  because  it  often  happens 
that  too  much  heat  is  removed.  If  the  cylinder 
is  too  cool  the  pressure  of  the  gas  expanding 
during  a  power  stroke  will  be  lessened  by  a 
large  amount  of  its  heat  entering  the  cylinder 
wall,  for  it  will  be  remembered  that  the  pressure 
exerted  by  a  gas  is  governed  largely  by  tem- 
perature. From  this  it  can  be  seen  that  there 
is  a  great  necessity  for  cooling,  but  to  get  the 
maximum  efficiency  from  an  engine  it  likewise 
is  necessary  to  avoid  over  cooling. 

Heat  may  be  conducted  from  a  cylinder  by 

[80J 


COOLING 

water  or  by  air.  Fixed-cylinder  engines  with  one 
or  two  exceptions  are  always  water  cooled, 
while  rotary  engines  are  invariably  air  cooled. 
The  advantages  in  air  cooling  are  a  decrease  in 
weight  and  the  avoidance  of  a  circulating  sys- 
tem that  can  be  pierced  by  bullets.  However, 
uniform  cooling  can  best  be  accomplished  by 
water.  The  most  important  point  to  note  re- 
garding air-cooled  engines  is  that  the  cylinders 
are  supplied  with  cooling  flanges  which  in- 
crease the  surface  from  which  the  heat  may  be 
radiated. 

Water-cooled  engines  make  use  of  a  radia- 
tor, usually  cellular  though  sometimes  tubular 
in  construction,  and  water  jackets  upon  the 
cylinders.  Water  is  supplied  to  the  base  of  the 
jackets  and  moves  upward  over  the  heads  of 
cylinders.  At  the  top  of  each  cylinder  it  is  con- 
ducted to  the  top  of  the  radiator  where  it  is 
cooled  and  consequently  tends  to  move  to- 
ward the  base  of  the  radiator.  From  this  point 
the  relatively  cool  water  is  drawn  to  the  pump 
which  delivers  it  to  the  water  jackets  to  be 
heated  again.  In  this  way  the  water  is  circu- 
lated by  the  pump  in  the  same  direction  that 
the  constant  heating  and  cooling  would  cause 
it  to  travel.  A  thermosyphon  system  would  be 

[81] 


ELEMENTS  OF  AVIATION  ENGINES 

one  depending  wholly  upon  automatic  circula- 
tion from  this  source.  Oftentimes  to  prevent 
condensation  within  the  intake  manifold  a 
little  of  the  hot  water  that  is  about  to  enter  the 
top  of  the  radiator  is  led  through  a  jacket  on 
the  manifold  and  from  there  to  the  intake  of 
the  pump. 

The  type  of  water  pump  used  on  aviation 
engines  is  with  few  exceptions  the  centrifugal 
pump.  In  cases  where  these  are  not  used  the 
gear  pump  is  employed.  In  a  centrifugal  pump 
the  water  is  led  into  the  center  of  a  circular 
chamber  in  which  revolve  several  blades  on  a 
common  shaft.  A  whirling  motion  of  the  water 
is  secured  enabling  it  to  be  led  away  at  a 
tangent  to  the  circular  chamber  under  pres- 
sure. One  advantage  of  a  centrifugal  pump 
over  a  gear  pump  is  that  the  water  may  circu- 
late through  the  pump  after  it  has  stopped 
operating.  When  a  gear  pump  ceases  to  oper- 
ate the  water  system  is  blocked. 

Hose  connections  are  used  on  the  water  lines 
between  the  engine  and  the  radiator.  These 
prevent  many  of  the  vibrations  of  the  engine 
from  reaching  the  radiator.  In  making  hose 
connections,  especially  on  the  line  leading  to 
the  intake  of  the  pump,  care  should  be  taken 

[82] 


CENTRIFUGAL  PUMP 


COOLING 

not  to  have  more  than  one  and  one-half  inches 
of  hose  exposed  to  the  water,  as  there  is  danger 
of  the  hose  weakening  and  lessening  the  flow 
by  its  being  drawn  together. 

When  a  plane  is  to  be  used  in  winter  weather 
or  when  it  is  required  to  fly  at  a  great  altitude, 
anti-freezing  mixtures  may  be  used.  The  best 
one  consists  of  17  per  cent  alcohol,  17  per  cent 
glycerine,  and  66  per  cent  water.  Such  a  mix- 
ture will  be  suitable  at  a  temperature  as  low  as 
15°  below  zero.  Although  this  mixture  is  far 
superior  to  salt  solutions  it  is  not  perfectly  sat- 
isfactory, because  after  being  heated  there  is 
always  an  uncertainty  as  to  the  quantity  of 
alcohol. 


[83] 


CHAPTER  IX 
ROTARY  ENGINES 

A  ROTARY  engine  is  one  in  which  the  cylin- 
ders and  crank  case  revolve  about  a  sta- 
tionary crank  shaft.  The  common  type  is  that 
having  the  cylinders  placed  in  the  same  plane 
and  radiating  at  equal  intervals  from  a  com- 
mon center.  The  center  of  rotation  is  the  center 
of  the  crank  shaft.  By  placing  all  the  cylin- 
ders in  the  same  vertical  plane  a  crank  shaft  of 
only  one  throw  can  be  used,  which  means  a 
centralizing  of  the  forces  exerted  upon  the 
crank  shaft.  With  the  throw  placed  vertically 
upward,  the  pistons  will  reach  top  center  when 
their  respective  cylinders  are  directly  above 
the  single  crank  pin.  If  an  explosion  occurs 
within  a  cylinder  when  it  is  at  top  position  the 
effect  will  be  to  increase  the  combustion  space 
by  revolving  the  cylinder  farther  away  from 
the  crank  pin,  which  allows  the  piston  to  move 
away  from  the  cylinder  head.  The  force  that 
revolves  the  cylinder  is  the  force  exerted  by 
the  piston  upon  the  cylinder  wall. 

Considering  that  the  cylinders  revolve  in- 
stead of  the  crank  shaft,  it  will  at  once  be  ap- 

[84] 


\ 


\ 


\ 


\ 


\ 


I 

/ 


DIAGRAM  TO  ILLUSTRATE  THE  PRINCIPLE 
OF  A  ROTARY  ENGINE 


ROTARY  ENGINES 

parent  that  rotary  engines  will  differ  from 
fixed  cylinder  engines  in  the  manner  of  sup- 
port, the  way  the  cylinders  are  retained,  the 
way  the  gasoline  mixture  is  supplied  to  the 
cylinders,  the  way  the  valves  are  operated,  and 
the  way  electricity  is  made  to  reach  the  spark 
plugs. 

Briefly,  rotary  engines  are  supported  by  two 
plates  holding  the  rear  end  of  the  crank  shaft 
which  allows  the  engine  to  overhang  its  sup- 
port. The  plate  nearest  the  engines,  which 
carries  the  magnetos  and  pumps  is  called  the 
bearer  plate.  The  rear  one  is  the  centralizing 
plate.  The  cylinders  are  retained  by  screwing 
them  into  the  crank  case  and  locking  them 
with  a  lock  nut,  or  by  having  the  crank  case 
made  in  two  parts  and  clamping  their  flanged 
bases  between  the  two  halves  of  the  crank  case. 
In  rotary  engines  the  crank  case  is  used  to 
store  the  gas  which  is  conducted  to  the  cylinder 
either  by  means  of  ports  in  the  cylinder  wall  or 
by  individual  manifolds  from  the  crank  case 
to  the  intake  valves  in  the  heads  of  the  cylin- 
ders. The  valves  are  operated  by  rods  and 
rocker  arms  with  sometimes  an  attempt  to  ful- 
crum the  rocker  arm  at  such  a  point  that  the 
centrifugal  force  acting  upon  the  rod  will  be 

[85] 


ELEMENTS  OF  AVIATION  ENGINES 

counterbalanced  by  that  acting  upon  the 
rocker  arm  and  the  valve.  Cam  plates  or  cam 
packs  revolving  on  the  stationary  crank  shaft 
are  used  to  operate  the  rods.  Electricity  is  led 
to  the  spark  plugs  by  means  of  bare  brass  wires 
drawn  taut  to  segments  imbedded  in  the  re- 
volving thrust  plate.  A  stationary  brush 
protruding  through  the  bearer  plate  has  con- 
nection with  the  magneto  and  comes  in  contact 
with  each  segment  as  the  engine  revolves. 

The  demand  for  increased  power  has  led  to 
the  design  of  rotary  engines  with  an  additional 
bank  of  cylinders  behind  the  first  bank.  The 
same  general  construction  is  used  except  that 
a  crank  shaft  of  two  throws  must  be  used  with 
this  type  of  engine.  Those  having  one  bank 
always  have  an  odd  number  of  cylinders  while 
the  two-bank  engines  have  an  even  number. 
However,  this  even  number  is  occasioned  by 
an  odd  number  of  cylinders  being  on  each  of 
the  two  banks.  The  reason  for  an  odd  number 
of  cylinders  is  to  allow  for  an  equal  spacing  of 
the  power  impulses.  These  engines,  being  four- 
stroke  cycle  engines,  will  have  all  cylinders 
function  once  in  two  revolutions.  By  using  an 
odd  number  it  is  possible  to  fire  alternate  cy- 
linders as  they  come  to  top  position,  which 

[86] 


ROTARY  ENGINES 

results  in  all  cylinders  having  a  chance  to  work 
once  during  two  revolutions  and  still  allow 
another  cycle  to  be  started  without  any  inter- 
ruption. The  cylinders  are  numbered  in  a 
clockwise  direction  viewed  from  the  propeller 
end. 

While  rotary  engines  are  made  almost  en- 
tirely of  steel,  as  a  general  rule  they  develop 
much  more  power  for  their  weight  than  fixed- 
cylinder  engines.  This  is  due  largely  to  the 
short  crank  shaft,  the  short  crank  case,  and  the 
fact  that  they  are  air  cooled  instead  of  water 
cooled.  On  account  of  the  difficulty  in  supply- 
ing the  revolving  cylinders  with  gas,  these  en- 
gines use  much  more  fuel  proportionately  than 
do  fixed-cylinder  engines.  Due  to  their  light 
weight  they  have  become  very  popular  for 
speed  scout  work,  where  brief  but  rapid  flights 
are  necessary.  For  great  distances  they  are  not 
looked  upon  with  much  favor  because  of  the 
great  quantity  of  fuel  that  must  be  carried  to 
answer  the  engine's  needs.  For  stunt  flying 
rotary  engines  are  admirably  suited,  on  ac- 
count of  their  ability  to  work  perfectly  in  any 
position.  In  any  comparison  of  rotary  and 
fixed-cylinder  engines  it  must  not  be  lost  sight 
of  that  rotary  engines,  due  to  their  radial  form, 

[871 


ELEMENTS  OF  AVIATION  ENGINES 

offer  the  more  head  resistance.  It  is  difficult  to 
meet  the  demand  for  increased  power  with 
rotary  type  engines  except  in  those  cases  where 
two-bank  engines  are  used.  An  attempt  to 
lengthen  the  stroke  results  in  longer  cylinders, 
which  means  more  centrifugal  force  will  be 
developed.  The  bore  has  limitations,  because 
the  sum  of  the  diameters  of  the  cylinders  must 
be  approximately  the  same  as  the  circumfer- 
ence of  the  crank  case.  The  compression  can 
not  be  greatly  increased,  because  the  resulting 
increase  in  temperature  is  more  than  can  be 
satisfactorily  cared  for  by  air  cooling. 

The  Gnome  "monosoupape"  is  a  well-known 
rotary  engine  which  has  attracted  wide  atten- 
tion. It  is  made  in  two  sizes  having  one  and 
two  banks  of  cylinders.  The  nine-cylinder  en- 
gine is  the  100  H.P.  Gnome,  while  the  eighteen- 
cylinder  one  is  the  180  H.P.  Except  for  the 
number  of  cylinders  the  two  engines  are  very 
similar.  In  entering  into  a  description  it  is 
sufficient  to  take  up  the  nine-cylinder  engine. 

In  any  rotary  engine  great  pains  must  be 
taken  to  prevent  centrifugal  force  from  carry- 
ing the  cylinders  away.  In  the  nine-cylinder 
Gnome  the  crank  case  is  split  into  two  circular 
halves,  and  the  cylinders  are  clamped  between 

[88] 


ROTARY  ENGINES 

the  two  parts.  The  cylinders,  which  are  ma- 
chined from  billets  of  steel,  are  drilled  so  that 
a  ring  of  small  ports  is  located  near  the  base  of 
each  cylinder.  These  serve  as  an  inlet  valve, 
for,  as  a  piston  goes  to  bottom  center,  the  ports 
are  uncovered  and  direct  connection  is  made 
between  the  interior  of  the  crank  case  and  the 
space  beyond  the  piston  head.  The  head  of  the 
cylinder  is  supplied  with  a  large  exhaust  valve 
which  gives  rise  to  the  name  "monosoupape," 
French  for  single  valve. 

The  valve  timing  is  novel  inasmuch  as  it  is  a 
four-stroke  cycle  engine  using  a  two-stroke 
cycle  method  of  admitting  the  charge.  The 
spark  occurs  18°  of  the  engine's  rotation  before 
top  center.  The  power  stroke  is  interrupted 
85°  past  top  center  by  the  opening  of  the  ex- 
haust valve.  This  valve  remains  open  395°  or 
120°  past  top  center,  allowing  all  the  burned 
gas  to  be  expelled  and  a  supply  of  air  to  be 
drawn  in  during  the  120°  it  remains  open  while 
the  piston  is  going  down.  After  the  exhaust 
valve  is  closed  the  downward  motion  of  the 
piston  tends  to  create  a  partial  vacuum  within 
the  cylinder  so  that  when  the  intake  ports  are 
uncovered  20°  before  bottom  center,  a  very 
rich  mixture  that  is  stored  in  the  crank  case 

[89] 


ELEMENTS  OF  AVIATION  ENGINES 

will  rush  into  the  cylinder.  The  gas  enters 
during  the  40°  that  the  ports  are  uncovered 
and  mixes  with  the  air  that  has  been  drawn  in 
through  the  exhaust  valve.  A  suitable  mixture 
is  thus  formed  which  is  compressed  and  ignited 
18°  before  top  center.  The  reason  for  the  early 
opening  of  the  exhaust  valve  is  to  secure  the 
same  pressure  within  the  cylinder  as  that  with- 
in the  crank  case  when  the  intake  ports  are 
uncovered  on  the  power  stroke. 

The  rich  mixture  held  in  the  crank  case  is 
formed  by  gasoline  being  sprayed  from  a  noz- 
zle connected  to  the  pipe  that  extends  within 
the  hollow  crank  shaft.  The  gasoline  which  is 
under  a  pressure  of  five  pounds  is  led  through 
a  shut-off  valve  located  in  the  cock  pit  for  the 
pilot  to  control.  Obviously  the  range  of  speed 
is  greatly  limited  by  this  means  of  control,  be- 
cause too  lean  a  mixture  is  apt  to  result  in 
back-firing  and  ruining  the  engine.  A  safer 
way  to  reduce  the  speed  is  to  make  the  mixture 
too  rich. 

Electricity  is  supplied  to  the  spark  plugs  by 
a  high-tension  magneto  located  on  the  bearer 
plate,  with  a  stationary  brush  bearing  upon 
the  segments  that  revolve  at  engine  speed. 
The  magneto  must  turn  two  and  one-fourth 

[90] 


ROTARY  ENGINES 

times  engine  speed  since  this  is  a  four-stroke 
cycle  engine  of  nine  cylinders.  Due  to  the  fact 
that  an  ordinary  magneto  supplies  but  two 
sparks  per  revolution  the  magneto  does  not 
furnish  a  spark  every  time  the  brush  is  on  a 
segment,  but  with  a  2  J4  gearing  it  is  capable  of 
furnishing  sparks  for  alternate  segments.  This 
is  what  is  needed  to  obtain  the  right  firing 
order. 

The  connecting  rods  are  made  to  work  upon 
a  hub  that  revolves  upon  the  crank  pin.  One 
connecting  rod  called  the  master  rod,  is  made 
integral  with  the  hub  to  maintain  its  proper 
rate  of  rotation.  The  eight  other  connecting 
rods  are  pinned  to  the  hub.  The  master  con- 
necting rod  prevents  the  lower  ends  of  the  con- 
necting rods  from  moving  too  far  from  their 
respective  cylinders.  In  order  that  the  hub 
may  be  mounted  upon  the  crank  pin  it  is 
necessary  for  the  crank  shaft  to  be  made  in 
two  pieces.  From  this  it  follows  that  the  crank 
shaft  will  be  weakened  so  that  the  thrust  of  the 
propeller  must  be  transmitted  through  the 
crank  case  to  a  thrust  bearing  at  the  rear  of 
the  engine. 

The  pistons  of  the  Gnome  engine  are  made 
of  cast  iron  with  the  piston  bosses  attached  to 

[91] 


ELEMENTS  OF  AVIATION  ENGINES 

the  concave  piston  head.  The  trailing  edges  of 
the  skirts  are  cut  away  to  prevent  piston  in- 
terference. The  surface  on  the  leading  edge 
is  not  reduced  as  it  will  be  remembered  that  it 
is  the  force  of  the  piston  against  the  cylinder 
wall  that  causes  the  engine  to  turn.  On  ac- 
count of  the  leading  edge  of  a  cylinder  coming 
in  contact  with  more  air  than  the  trailing  edge, 
the  expansion  of  a  cylinder  will  be  slightly 
irregular.  This  makes  necessary  a  compression 
ring  that  will  conform  to  the  irregularity.  A 
flexible  L-shaped  bronze  ring  known  as  an 
"obturator*'  is  used.  This  ring  is  retained  in  a 
groove  very  near  the  piston  head  by  means  of 
a  steel  packing  ring.  The  gap  in  the  "obtura- 
tor" is  placed  on  the  leading  edge  where  there 
is  the  least  amount  of  clearance.  The  piston  is 
also  supplied  with  a  cast-iron  oiling  ring. 

The  exhaust  valves  are  operated  by  rocker 
arms  and  push  rods,  which,  with  the  tappets, 
radiate  spirally  from  the  cam  pack  located  at 
the  propeller  end  of  the  engine.  The  nine  cams 
on  the  cam  pack  are  designed  with  197^° 
faces  on  account  of  the  exhaust  valve  being 
held  open  for  395°.  The  cam  pack  is  made  to 
turn  on  the  stationary  crank  shaft  at  half 
engine  speed  by  a  system  of  six  planetary 

[92] 


ROTARY  ENGINES 

gears.  A  thirty-tooth  gear  held  rigidly  upon 
the  crank  shaft  meshes  with  two  thirty-tooth 
gears  pinned  to  the  crank  case.  Each  of  the 
revolving  thirty-tooth  gears  has  a  twenty- 
tooth  gear  secured  rigidly  to  it,  and  it  is  the 
twenty-tooth  gears  that  mesh  with  one  having 
forty  teeth  attached  to  the  cam  pack.  The  re- 
duction of  two  to  one  is  thus  secured. 

Castor  oil  is  delivered  to  two  tubes  within 
the  crank  shaft  by  a  double  plunger  pump 
located  upon  the  bearer  plate.  Pulsators  are 
used  to  indicate  the  operations  of  the  pumps. 
The  oil  from  one  tube  goes  to  lubricate  the 
front  and  rear  bearings  and  the  cam  pack. 
The  oil  from  the  second  tube  is  used  to  oil  the 
connecting  rod  assembly  and  wrist  pins.  Spray 
from  the  connecting  rod  assembly  comes  in 
contact  with  the  cylinder  walls.  Centrifugal 
force  which  carries  the  oil  out  the  exhaust 
valves  prevents  the  use  of  a  circulating  sys- 
tem. Due  to  oil  being  carried  toward  the 
cylinder  head,  it  is  impractical  to  place  the 
spark  plugs  in  the  head.  They  are  placed  on 
the  leading  edge  where  they  are  less  likely  to 
become  fouled. 

The  Le  Rhone  engine  with  its  threaded 
cylinders  and  peculiar  valve  operation  has 

[931 


ELEMENTS  OF  AVIATION  ENGINES 

attracted  a  great  deal  of  attention.  Gas  is  sup- 
plied to  the  crank  case  by  a  crude  carburetor 
attached  to  the  rear  end  of  the  hollow  crank 
shaft.  A  throttle  and  metering  pin  are  used  in 
the  carburetor  allowing  a  slightly  wider  range 
of  speed  than  can  be  obtained  with  the  Gnome. 
The  inlet  valve  being  located  in  the  cylinder 
head,  a  separate  manifold  is  used  to  conduct 
the  gas  from  the  crank  case  to  each  cylinder. 

The  cylinders  are  of  steel,  and,  being  threaded 
at  the  base,  they  are  retained  by  being  screwed 
into  the  crank  case  and  locked  with  a  large 
lock  nut.  This  design  permits  the  compression 
to  be  changed  by  screwing  the  cylinders  in  or 
out  as  the  case  may  be.  A  cast-iron  liner 
shrunk  into  each  cylinder  does  much  to  pre- 
vent irregular  expansion  from  interfering  with 
the  piston  rings.  No  * 'obturator'*  is  used  on 
the  Le  Rhone. 

The  inlet  and  the  exhaust  valves  located  in 
the  cylinder  head  are  operated  by  a  single  rod 
for  each  cylinder.  A  rocker  arm  is  attached  to 
each  end  of  the  rod.  The  base  rocker  arm  is 
fulcrumed  to  the  crank  case  with  both  ends 
supplied  with  rollers,  each  bearing  upon  a 
separate  cam  plate.  These  cam  plates  have 
five  cams  upon  each  one  and  are  so  constructed 

[94] 


ROTARY  ENGINES 

that  when  one  end  of  the  rocker  arm  is  forced 
up  by  one  plate  the  other  end  sinks  into  a  de- 
pression on  the  second  plate.  In  this  way  the 
rod  is  used  both  as  a  push  rod  to  open  the  ex- 
haust valve  and  a  pull  rod  to  open  the  inlet 
valve.  Since  each  cam  plate  has  five  cams  they 
are  revolved  at  nine- tenths  engine  speed.  This 
rate  is  necessary  because  the  valves  must  open 
nine  times  in  two  revolutions  of  the  engine, 
and  in  two  revolutions  of  the  cam  pack  ten 
cams  come  into  position. 

The  connecting  rods  are  designed  with  shoes 
at  their  large  ends.  The  hub  on  the  crank  pin 
is  made  up  of  two  discs  each  having  three 
grooves  to  receive  the  connecting  rod  shoes. 
The  discs  are  clamped  together  and  hold  the 
connecting  rods  between  them.  Each  groove 
holds  three  shoes,  and  being  a  nine-cylinder 
engine,  it  follows  that  the  connecting  rods  will 
be  of  three  lengths.  With  this  construction  no 
master  rod  is  necessary. 


[95] 


CHAPTER  X 
THE  LIBERTY  MOTOR 

THE  LIBERTY  MOTOR,  which  represents  the 
latest  development  in  aviation  engines, 
is  not  known  in  detail  by  many  at  present. 
Due  to  the  discretion  of  the  War  Department, 
little  if  anything  could  be  learned  regarding  it 
during  the  time  that  the  first  engines  were 
being  built  and  tested.  Now  that  its  success 
is  assured,  the  Committee  on  Public  Informa- 
tion has  given  the  writer  permission  to  print 
the  facts  set  forth  in  a  recent  number  of  "The 
Official  Bulletin/'  The  following  paragraphs 
are  authorized  by  the  War  Department  and 
the  Committee  on  Public  Information: 

"The  designs  of  the  parts  of  the  Liberty 
engine  were  based  on  the  following: 

"Cylinders. — The  designers  of  the  cylinders 
for  the  Liberty  engine  followed  the  practice 
used  in  the  German  Mercedes,  English  Rolls 
Royce,  French  Lorraine,  Dietrich,  and  Italian 
Isotta  Fraschini  before  the  war  and  during  the 
war.  The  cylinders  are  made  of  steel  inner 
shells  surrounded  by  pressed-steel  water  jack- 
ets. The  Packard  Co.  by  long  experiment  had 

[96] 


THE  LIBERTY  MOTOR 

developed  a  method  of  applying  these  steel 
water  jackets. 

"The  valve  cages  are  drop  forgings  welded 
into  the  cylinder  head.  The  principal  depar- 
ture from  European  practices  is  in  the  location 
of  the  holding-down  flange,  which  is  several 
inches  above  the  mouth  of  the  cylinder,  and 
the  unique  method  of  manufacture  evolved  by 
the  Ford  Co.  The  output  is  now  approxi- 
mately 1,700  cylinder  forgings  per  day. 

"Cam  shaft  and  valve  mechanism  above 
cylinder  heads. — The  design  of  the  above  is 
based  on  the  Mercedes,  but  was  improved 
for  automatic  lubrication  without  wasting  oil 
by  the  Packard  Motor  Car  Co. 

"Cam-shaft  drive. — The  cam-shaft  drive 
was  copied  almost  entirely  from  the  Hall-Scott 
motor;  in  fact,  several  of  the  gears  used  in  the 
first  sample  engines  were  supplied  by  the  Hall- 
Scott  Motor  Car  Co.  This  type  of  drive  is  used 
by  Mercedes,  Hispano-Suiza,  and  others. 

"Angle  between  cylinders. — In  the  Liberty 
the  included  angle  between  the  cylinders  is 
45°;  in  all  other  existing  twelve-cylinder  en- 
gines it  is  60°.  This  feature  is  new  with  the 
Liberty  engine,  and  was  adopted  for  the  pur- 
pose of  bringing  each  row  of  cylinders  nearer 

[97] 


ELEMENTS  OF  AVIATION  ENGINES 

the  vertical  and  closer  together,  so  as  to  save 
width  and  head  resistance.  By  the  narrow 
angle  greater  strength  is  given  to  the  crank 
case  and  vibration  is  reduced. 

" Electric  generator  and  ignition. — A  Delco 
ignition  system  is  used.  It  was  especially  de- 
signed for  the  Liberty  engine  to  save  weight 
and  to  meet  the  special  conditions  due  to  firing 
twelve  cylinders  with  an  included  angle  of  45°. 

"Pistons. — The  pistons  of  the  Liberty  en- 
gine are  of  Hall-Scott  design. 

"Connecting  rods. — Forked  or  straddle- type 
connecting  rods,  first  used  on  the  French  De 
Dion  car  and  on  the  Cadillac  motor  car  in  this 
country,  are  used. 

"Crank  shaft. — Crank  shaft  design  followed 
the  standard  twelve-cylinder  practice,  except 
as  to  oiling.  Crank  case  follows  standard  prac- 
tice. The  45°  angle  and  the  flange  location  on 
the  cylinders  made  possible  a  very  strong  box 
section. 

"Lubrication. — The  first  system  of  lubrica- 
tion followed  the  German  practice  of  using  one 
pump  to  keep  the  crank  case  empty,  deliver- 
ing into  an  outside  reservoir,  and  another 
pump  to  force  oil  under  pressure  to  the  main 
crank-shaft  bearings.  This  lubrication  system 

[98] 


THE  LIBERTY  MOTOR 

also  followed  the  German  practice  in  allowing 
the  overflow  in  the  main  bearings  to  travel  out 
the  face  of  the  crank  cheeks  to  a  scupper  which 
collected  this  excess  for  crank-pin  lubrication. 
This  is  very  economical  in  the  use  of  oil  and  is 
still  the  standard  German  practice. 

"The  present  system  is  similar  to  the  first 
practice,  except  that  the  oil,  while  under  pres- 
sure, is  not  only  fed  to  main  bearings  but 
through  holes  inside  of  crank  cheeks  to  crank 
pins,  instead  of  feeding  these  crank  pins 
through  scuppers.  The  difference  between 
the  two  oiling  systems  consists  of  carrying  oil 
for  the  crank  pins  through  a  hole  inside  the 
crank  cheek  instead  of  up  the  outside  face  of 
the  crank  cheek. 

"Propeller  hub.— The  Hall-Scott  propeller- 
hub  design  was  adapted  to  the  power  of  the 
Liberty  engine. 

"Water  pump. — The  Packard  type  of  water 
pump  was  adapted  to  the  Liberty. 

"Carburetor. — A  carburetor  was  developed 
by  the  Zenith  Co.  for  the  Liberty  engine. 

"Bore  and  stroke. — The  bore  and  stroke  of 
the  Liberty  engine  is  5  by  7  inches,  the  same 
as  the  Hall-Scott  A-5  and  A-7  engines,  and  as 
in  the  Hall-Scott  12-cylinder  engine. 

[99] 


ELEMENTS  OF  AVIATION  ENGINES 

"Remarks. — The  idea  of  developing  Liberty 
engines  of  4,  6,  8,  and  12  cylinders  with  the 
above  characteristics  was  first  thought  of 
about  May  25,  1917.  The  idea  was  developed 
in  conference  with  representatives  of  the  Brit- 
ish and  French  missions,  May  28  to  June  1, 
and  was  submitted  in  the  form  of  sketches  at 
a  joint  meeting  of  the  Aircraft  (Production) 
Board  and  the  Joint  Army  and  Navy  Tech- 
nical Board,  June  4.  The  first  sample  was  an 
eight-cylinder  model,  delivered  to  the  Bureau 
of  Standards  July  3,  1917.  The  eight-cylinder 
model,  however,  was  never  put  into  produc- 
tion, as  advices  from  France  indicated  that 
demands  for  increased  power  would  make  the 
eight-cylinder  model  obsolete  before  it  could 
be  produced. 

"WORK  ON  THE  12-CYLINDER  EN- 
GINE. 

"Work  was  then  concentrated  on  the  12- 
cylinder  engine,  and  one  of  the  experimental 
engines  passed  the  50-hour  test  August  25, 
1917. 

"After  the  preliminary  drawings  were  made, 
engineers  from  the  leading  engine  builders 
were  brought  to  the  Bureau  of  Standards, 
where  they  inspected  the  new  designs  and 

[100] 


THE  LIBERTY  MOTOR 

made  suggestions,  most  of  which  were  in- 
corporated in  the  final  design.  At  the  same 
time  expert  production  men  were  making  sug- 
gestions that  would  facilitate  production. 

'The  Liberty  12-cylinder  engine  passed  the 
50-hour  test,  showing  as  the  official  report  of 
August  25,  1917,  records  'that  the  fundamen- 
tal construction  is  such  that  very  satisfactory 
service  with  a  long  life  and  high  order  of  effi- 
ciency will  be  given  by  this  power  plant,  and 
that  the  design  has  passed  from  the  experi- 
mental stage  into  the  field  of  proven  engines'. 

"An  engine  committee  was  organized  in- 
formally, consisting  of  the  engineers  and  pro- 
duction managers  of  the  Packard,  Ford, 
Cadillac,  Lincoln,  Marmon,  and  Trego  com- 
panies. This  committee  met  at  frequent 
intervals,  and  it  is  to  this  group  of  men  that 
the  final  development  of  the  Liberty  engine 
is  largely  due." 


[101] 


INDEX 


INDEX 

PAGES 

ALTITUDE  COMPENSATION 56 

ANGLE  BETWEEN  CYLINDERS   ......  97 

AUXILIARY  AIR  VALVES 56 

BATTERY  IGNITION  SYSTEM 68 

BEARING  LINERS 31 

BERLING  MAGNETO 64 

BORE        .      .      ;     ...      .    '  ...    ' 20 

BOSCH  MAGNETO  .      .      ...      ...      .      ,      .  64 

BREATHERS     .      .      ...      .      .      .      .      .  25 

CAMSHAFTS  .......      ..      .      .      .      ...  36 

CASTOR  OIL    .      .      .      .„      .     .,      .      .      .      .  74 

CENTRIFUGAL  PUMP    .      ...      ...      .      .  82 

CLEARANCE  OF  PISTONS      .       .      ...,:.      .  27 

CLEARANCE  OF  VALVE  STEMS  .      «'•  "•      .      .      .  44 

CONNECTING  RODS      .      .      .      .      ;      .      .      .  29 

CRANKSHAFTS      .      .      .      .      *      *      .      .      .  30 

CYCLE      .      .      .      .      .      ..      ..      .      ;      .      .  8 

CYLINDERS     .      ...      .      ..      ....  22 

CYLINDER  NUMBERING      .      .      .      ...      .  33 

DIXIE  MAGNETO  ..,.,...      .      .      .  64 

EXHAUST  VALVES 39 

FIRE  POINT  OF  LUBRICATING  OILS 73 

FIRING  ORDERS .      .33 

FLASH  POINT  OF  LUBRICATING  OILS      ....  73 

FOUR-STROKE  CYCLE  ENGINES 9 

GEAR  OIL  PUMP 75 

GNOME  ENGINE 88 

HORSE-POWER 16 

[105] 


INDEX(  Continued] 

PAGES 

INDUCTOR  TYPE  MAGNETO 59 

INLET  VALVES 40 

LE  RHONE  ENGINE 93 

METERING  PINS    .       .      . 51 

NORMAL  AND  ANTI-NORMAL  ENGINES    ....  20 

OIL-COOLING  DEVICES  . 78 

PISTONS    .      .      .   '",     v     ......  25 

PISTON  DISPLACEMENT       .      ..      .      ..     -^     .      .  21 

PISTON  RINGS       .      .      .      .      ,.      ...     ^   '.      .  27 

PRIMARY  CURRENT     .      „*" '    .    '*      .-     ,      .      .  60 

PROPELLER  SPEED       ..     '%      ,      »•  '  • .      .      .      .  19 

PROPORTION  OF  GASOLINE  TO  AIR  .      .      «      ^    .  47 

SECONDARY  CURRENT 62 

SHUTTLE  TYPE  MAGNETO  .       .       .       ...      .      .  59 

SPEED  COMPENSATION        .    .  „•'      ,      .      ...  50 

STROKE    .      .      ^ .     ;      i      -      .      .-     .      .      .  10 

THERMOSYPHON  COOLING  SYSTEM  .      .      .      ^      .  81 

THRUST  BEARINGS       ,      +      ,      ,  .    .      .      „      .  35 

TWO-STROKE  CYCLE  ENGINES  .      .    '  .      .      ».     .  7 

VALVE  GRINDING  .      ,.      ..      ^      ./     .      .      .      .  40 

VALVE  TIMING      .      *      »      .      .      .      .      ,      .  43 

VANE  OIL  PUMP .      .      ^     .  76 

VIBRATION      .      ..      .      .      .      .      .      .      »      .  13 

VISCOSITY  OF  LUBRICATING  OILS    .  r-     .      .73 

WRIST  PINS    .      .      ...     \      ,-•    *   .*     >      .  28 

ZENITH  CARBURETOR  .  52 


[106] 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


STUDENT'S  NOTES 


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THIS  BOOK  ON   THE  DATE  DUE.      THE 
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DAY     AND     TO     $1.OO     ON     THE     SEVENTh 
OVERDUE. 


MAY    21  1944 


REC'D  LD 

DEC    51957 


OCT  14  v 


SEP  30 '6 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


